Why did removing 'python3' remove many other important packages?

What I did:

sudo apt-get remove python3

What I got in response :

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  bicyclerepair ca-certificates-mono cli-common dc extlinux gedit-common
  gir1.2-appindicator3-0.1 gir1.2-gmenu-3.0 gir1.2-grilo-0.2 gir1.2-gudev-1.0
  gir1.2-mediaart-2.0 gir1.2-rb-3.0 gir1.2-secret-1 gir1.2-tracker-1.0
  gir1.2-udisks-2.0 guile-2.0-libs intel-gpu-tools kdesudo libandroid-properties1
  libdbus-glib2.0-cil libdbus2.0-cil libgc1c2 libgconf2.0-cil libgdiplus
  libgkeyfile1.0-cil libglib2.0-cil libgnome-keyring1.0-cil libgpod-common libgpod4
  libgtk2.0-cil libhud2 libjs-sphinxdoc libjs-underscore libmessaging-menu0
  libmono-accessibility4.0-cil libmono-addins0.2-cil libmono-cairo4.0-cil
  libmono-corlib4.5-cil libmono-data-tds4.0-cil libmono-i18n-west4.0-cil
  libmono-i18n4.0-cil libmono-ldap4.0-cil libmono-posix4.0-cil
  libmono-security4.0-cil libmono-sharpzip4.84-cil libmono-sqlite4.0-cil
  libmono-system-componentmodel-dataannotations4.0-cil
  libmono-system-configuration4.0-cil libmono-system-core4.0-cil
  libmono-system-data4.0-cil libmono-system-design4.0-cil
  libmono-system-drawing4.0-cil libmono-system-enterpriseservices4.0-cil
  libmono-system-ldap4.0-cil libmono-system-numerics4.0-cil
  libmono-system-runtime-serialization-formatters-soap4.0-cil
  libmono-system-runtime-serialization4.0-cil libmono-system-security4.0-cil
  libmono-system-servicemodel-internals0.0-cil libmono-system-transactions4.0-cil
  libmono-system-web-applicationservices4.0-cil libmono-system-web-services4.0-cil
  libmono-system-web4.0-cil libmono-system-windows-forms4.0-cil
  libmono-system-xml-linq4.0-cil libmono-system-xml4.0-cil libmono-system4.0-cil
  libmono-webbrowser4.0-cil libnotify0.4-cil liboxideqt-qmlplugin liboxideqtcore0
  liboxideqtquick0 libpeas-1.0-0-python3loader libpython3-dev libpython3.5-dev
  libqt5feedback5 libqt5multimedia5 libqt5organizer5 libqt5positioning5
  libqt5quicktest5 libqt5test5 librhythmbox-core9 libsgutils2-2 libubuntugestures5
  libubuntutoolkit5 libunity-action-qt1 libunity-webapps0 libunwind8 libx86-1
  mono-4.0-gac mono-gac mono-runtime mono-runtime-common mono-runtime-sgen mscompress
  oxideqt-codecs plainbox-secure-policy pm-utils python-gconf python-kiwi python-vte
  python3.5-dev qml-module-qt-labs-folderlistmodel qml-module-qt-labs-settings
  qml-module-qtfeedback qml-module-qtgraphicaleffects qml-module-qttest
  qml-module-ubuntu-components qml-module-ubuntu-layouts
  qml-module-ubuntu-onlineaccounts qml-module-ubuntu-performancemetrics
  qml-module-ubuntu-test qml-module-ubuntu-web qmlscene
  qtdeclarative5-accounts-plugin qtdeclarative5-dev-tools
  qtdeclarative5-qtquick2-plugin qtdeclarative5-test-plugin
  qtdeclarative5-ubuntu-ui-toolkit-plugin qtdeclarative5-unity-action-plugin
  rhythmbox-data suru-icon-theme syslinux syslinux-common syslinux-legacy tcl tk
  ubuntu-mobile-icons ubuntu-ui-toolkit-theme unetbootin-translations
  unity-webapps-qml unity-webapps-service vbetool vim-addon-manager webapp-container
  webbrowser-app
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
  kdesudo
The following packages will be REMOVED:
  aisleriot apparmor apport apport-gtk aptdaemon apturl apturl-common atom bpython3
  checkbox-converged checkbox-gui classicmenu-indicator command-not-found compiz
  compiz-gnome dh-python docky firefox foomatic-db-compressed-ppds gconf-editor
  gconf2 gdebi gdebi-core gdm3 gedit gir1.2-ibus-1.0 gksu gnome-menus gnome-music
  gnome-orca gnome-shell gnome-terminal hplip hplip-data ibus ibus-table
  idle-python3.5 idle3 language-selector-common language-selector-gnome
  libbonoboui2-0 libgksu2-0 libgnome-2-0 libgnome2-0 libgnome2-bin libgnome2-common
  libgnomeui-0 libgnomevfs2-0 libgnomevfs2-common libpurple-bin lsb-release
  mysql-server mysql-server-5.7 nautilus-share onboard onboard-data openprinting-ppds
  pida plainbox-provider-checkbox plainbox-provider-resource-generic
  plymouth-theme-ubuntu-gnome-text plymouth-theme-ubuntu-text printer-driver-foo2zjs
  printer-driver-foo2zjs-common printer-driver-postscript-hp printer-driver-ptouch
  printer-driver-pxljr pyotherside python3 python3-apport python3-apt
  python3-aptdaemon python3-aptdaemon.gtk3widgets python3-aptdaemon.pkcompat
  python3-argcomplete python3-argh python3-blessings python3-blinker python3-brlapi
  python3-bs4 python3-cairo python3-cffi-backend python3-chardet
  python3-checkbox-support python3-commandnotfound python3-cryptography python3-cups
  python3-cupshelpers python3-curtsies python3-dbus python3-debian python3-defer
  python3-dev python3-distupgrade python3-feedparser python3-gdbm python3-gi
  python3-gi-cairo python3-greenlet python3-guacamole python3-html5lib
  python3-httplib2 python3-idna python3-jinja2 python3-jwt python3-louis python3-lxml
  python3-mako python3-markupsafe python3-mysqldb python3-oauthlib python3-padme
  python3-pathtools python3-pexpect python3-pil python3-pip python3-pkg-resources
  python3-plainbox python3-problem-report python3-ptyprocess python3-pyasn1
  python3-pyatspi python3-pycurl python3-pygments python3-pyparsing python3-renderpm
  python3-reportlab python3-reportlab-accel python3-requests python3-setuptools
  python3-six python3-software-properties python3-speechd python3-systemd python3-tk
  python3-uno python3-update-manager python3-urllib3 python3-virtualenv
  python3-watchdog python3-wcwidth python3-wheel python3-xdg python3-xkit
  python3-xlsxwriter python3-yaml qml-module-io-thp-pyotherside rhythmbox
  rhythmbox-plugin-zeitgeist rhythmbox-plugins sessioninstaller snapd
  software-properties-common software-properties-gtk ssh-import-id
  system-config-printer-common system-config-printer-gnome system-config-printer-udev
  totem-plugins ubuntu-core-launcher ubuntu-desktop ubuntu-drivers-common
  ubuntu-minimal ubuntu-release-upgrader-core ubuntu-release-upgrader-gtk
  ubuntu-standard ubuntu-system-service ufw unattended-upgrades unetbootin unity
  unity-control-center unity-control-center-signon unity-lens-photos
  unity-scope-calculator unity-scope-chromiumbookmarks unity-scope-colourlovers
  unity-scope-devhelp unity-scope-firefoxbookmarks unity-scope-gdrive
  unity-scope-home unity-scope-manpages unity-scope-openclipart unity-scope-texdoc
  unity-scope-tomboy unity-scope-virtualbox unity-scope-yelp unity-scope-zotero
  unity-tweak-tool unity-webapps-common update-manager update-manager-core
  update-notifier update-notifier-common usb-creator-common usb-creator-gtk vim-gnome
  xdiagnose
The following NEW packages will be installed:
  kdesudo
0 upgraded, 1 newly installed, 198 to remove and 3 not upgraded.
Need to get 26.1 kB of archives.
After this operation, 414 MB disk space will be freed.
Do you want to continue? [Y/n] y
Get:1 http://in.archive.ubuntu.com/ubuntu xenial/universe amd64 kdesudo amd64 3.4.2.4+repack-2ubuntu4 [26.1 kB]
Fetched 26.1 kB in 1s (13.3 kB/s)
(Reading database ... 422256 files and directories currently installed.)
Removing aisleriot (1:3.18.2-1ubuntu1) ...
Removing mysql-server (5.7.12-0ubuntu1.1) ...
Removing mysql-server-5.7 (5.7.12-0ubuntu1.1) ...
update-alternatives: using /etc/mysql/my.cnf.fallback to provide /etc/mysql/my.cnf (my.cnf) in auto mode
Removing snapd (2.0.9) ...
Warning: Stopping snapd.service, but it can still be activated by:
  snapd.socket
Removing ubuntu-core-launcher (1.0.27.1) ...
Removing apparmor (2.10.95-0ubuntu2) ...
Removing apport-gtk (2.20.1-0ubuntu2.1) ...
Removing apport (2.20.1-0ubuntu2.1) ...
Removing ubuntu-desktop (1.361) ...
Removing language-selector-gnome (0.165.3) ...
Removing sessioninstaller (0.20+bzr150-0ubuntu4.1) ...
Removing aptdaemon (1.1.1+bzr982-0ubuntu14) ...
Removing nautilus-share (0.7.3-2ubuntu1) ...
Removing apturl (0.5.2ubuntu11.1) ...
Removing apturl-common (0.5.2ubuntu11.1) ...
Removing atom (1.8.0-1~webupd8~0) ...
Removing bpython3 (0.15-2) ...
Removing checkbox-gui (1.2.4-0ubuntu1) ...
Removing checkbox-converged (1.2.4-0ubuntu1) ...
Removing classicmenu-indicator (0.10.1-0ubuntu1) ...
Removing command-not-found (0.3ubuntu16.04.2) ...
Removing unity-tweak-tool (0.0.7ubuntu2) ...
Removing unity (7.4.0+16.04.20160526.1-0ubuntu1) ...
Removing gdm3 (3.18.3-0ubuntu2) ...
Please be sure to run "dpkg-reconfigure lightdm".
Removing compiz (1:0.9.12.2+16.04.20160526-0ubuntu1) ...
Removing compiz-gnome (1:0.9.12.2+16.04.20160526-0ubuntu1) ...
Removing gnome-shell (3.18.5-0ubuntu0.1) ...
Removing software-properties-gtk (0.96.20.2) ...
Removing software-properties-common (0.96.20.2) ...
Removing python3-software-properties (0.96.20.2) ...
Removing python3-uno (1:5.1.4-0ubuntu1) ...
Removing python3-pip (8.1.1-2ubuntu0.1) ...
Removing docky (2.2.1.1-1) ...
Removing firefox (47.0+build3-0ubuntu0.16.04.1) ...
Removing foomatic-db-compressed-ppds (20160212-0ubuntu1) ...
Removing gconf-editor (3.0.1-2ubuntu1) ...
Removing unetbootin (608-1) ...
Removing gdebi (0.9.5.7ubuntu1) ...
Removing gksu (2.0.2-9ubuntu1) ...
Removing libgksu2-0 (2.0.13~pre1-6ubuntu8) ...
Removing pida (0.5.1-6) ...
Removing vim-gnome (2:7.4.1689-3ubuntu1.1) ...
update-alternatives: using /usr/bin/vim.basic to provide /usr/bin/vi (vi) in auto mode
update-alternatives: using /usr/bin/vim.basic to provide /usr/bin/view (view) in auto mode
update-alternatives: using /usr/bin/vim.basic to provide /usr/bin/ex (ex) in auto mode
update-alternatives: using /bin/nano to provide /usr/bin/editor (editor) in auto mode
update-alternatives: using /usr/bin/vim.basic to provide /usr/bin/rvim (rvim) in auto mode
update-alternatives: using /usr/bin/vim.basic to provide /usr/bin/rview (rview) in auto mode
update-alternatives: using /usr/bin/vim.basic to provide /usr/bin/vimdiff (vimdiff) in auto mode
update-alternatives: using /usr/bin/vim.basic to provide /usr/bin/vim (vim) in auto mode
Removing libgnomeui-0:amd64 (2.24.5-3.1) ...
Removing gdebi-core (0.9.5.7ubuntu1) ...
Removing gedit (3.18.3-0ubuntu4) ...
Removing unity-control-center-signon (0.1.8+16.04.20160201-0ubuntu1) ...
Removing unity-control-center (15.04.0+16.04.20160413-0ubuntu3) ...
Removing ibus-table (1.9.1-3ubuntu2) ...
Removing ibus (1.5.11-1ubuntu2) ...
Removing gir1.2-ibus-1.0:amd64 (1.5.11-1ubuntu2) ...
Removing gnome-menus (3.13.3-6ubuntu3) ...
Removing gnome-music (3.18.2-1ubuntu1) ...
Removing gnome-orca (3.18.2-1ubuntu3) ...
Removing gnome-terminal (3.18.3-1ubuntu1) ...
update-alternatives: using /usr/bin/lxterm to provide /usr/bin/x-terminal-emulator (x-terminal-emulator) in auto mode
Removing printer-driver-postscript-hp (3.16.3+repack0-1) ...
Removing hplip (3.16.3+repack0-1) ...
Removing hplip-data (3.16.3+repack0-1) ...
dpkg: warning: while removing hplip-data, directory '/usr/share/hplip/fax' not empty so not removed
dpkg: warning: while removing hplip-data, directory '/usr/share/hplip/pcard' not empty so not removed
dpkg: warning: while removing hplip-data, directory '/usr/share/hplip/base/pexpect' not empty so not removed
dpkg: warning: while removing hplip-data, directory '/usr/share/hplip/installer' not empty so not removed
dpkg: warning: while removing hplip-data, directory '/usr/share/hplip/copier' not empty so not removed
dpkg: warning: while removing hplip-data, directory '/usr/share/hplip/scan' not empty so not removed
dpkg: warning: while removing hplip-data, directory '/usr/share/hplip/prnt' not empty so not removed
dpkg: warning: while removing hplip-data, directory '/usr/share/hplip/ui4' not empty so not removed
Removing idle3 (3.5.1-3) ...
Removing idle-python3.5 (3.5.1-10) ...
Removing ubuntu-standard (1.361) ...
Removing language-selector-common (0.165.3) ...
Removing libbonoboui2-0:amd64 (2.24.5-3ubuntu1) ...
Removing libgnome2-0:amd64 (2.32.1-5ubuntu1) ...
Removing libgnome2-bin (2.32.1-5ubuntu1) ...
Removing libgnome-2-0:amd64 (2.32.1-5ubuntu1) ...
Removing libgnome2-common (2.32.1-5ubuntu1) ...
Removing libpurple-bin (1:2.10.12-0ubuntu5) ...
Removing onboard-data (1.2.0-0ubuntu5) ...
Removing onboard (1.2.0-0ubuntu5) ...
Removing openprinting-ppds (20160212-0ubuntu1) ...
Removing plainbox-provider-checkbox (0.25-1) ...
Removing plainbox-provider-resource-generic (0.23-1) ...
Removing plymouth-theme-ubuntu-gnome-text (16.04.4) ...
update-alternatives: using /usr/share/plymouth/themes/ubuntu-text/ubuntu-text.plymouth to provide /usr/share/plymouth/themes/text.plymouth (text.plymouth) in auto mode
update-initramfs: deferring update (trigger activated)
Removing plymouth-theme-ubuntu-text (0.9.2-3ubuntu13.1) ...
update-initramfs: deferring update (trigger activated)
Removing printer-driver-foo2zjs (20151024dfsg0-1ubuntu1) ...
Removing printer-driver-foo2zjs-common (20151024dfsg0-1ubuntu1) ...
Removing printer-driver-ptouch (1.4-1) ...
Removing printer-driver-pxljr (1.4+repack0-4) ...
Removing pyotherside (1.4.0-2) ...
Removing xdiagnose (3.8.4) ...
Removing python3-apport (2.20.1-0ubuntu2.1) ...
Removing ubuntu-drivers-common (1:0.4.17.1) ...
Removing python3-commandnotfound (0.3ubuntu16.04.2) ...
Removing system-config-printer-gnome (1.5.7+20160212-0ubuntu2) ...
Removing python3-aptdaemon.pkcompat (1.1.1+bzr982-0ubuntu14) ...
Removing python3-aptdaemon.gtk3widgets (1.1.1+bzr982-0ubuntu14) ...
Removing python3-aptdaemon (1.1.1+bzr982-0ubuntu14) ...
Removing python3-argcomplete (0.8.1-1ubuntu2) ...
Removing python3-watchdog (0.8.3-2) ...
Removing python3-argh (0.26.1-1) ...
Removing python3-curtsies (0.2.6-1) ...
Removing python3-blessings (1.6-2build1) ...
Removing unity-lens-photos (1.0+14.04.20140318-0ubuntu1) ...
Removing python3-oauthlib (1.0.3-1) ...
Removing python3-blinker (1.3.dfsg2-1build1) ...
Removing python3-brlapi (5.3.1-2ubuntu2.1) ...
Removing python3-bs4 (4.4.1-1) ...
Removing python3-gi-cairo (3.20.0-0ubuntu1) ...
Removing python3-cairo (1.10.0+dfsg-5build1) ...
Removing python3-cryptography (1.2.3-1) ...
Removing python3-cffi-backend (1.5.2-1ubuntu1) ...
Removing ssh-import-id (5.5-0ubuntu1) ...
Removing python3-plainbox (0.25-1) ...
Removing usb-creator-gtk (0.3.2) ...
Removing usb-creator-common (0.3.2) ...
Removing python3-checkbox-support (0.22-1) ...
Removing system-config-printer-udev (1.5.7+20160212-0ubuntu2) ...
Removing system-config-printer-common (1.5.7+20160212-0ubuntu2) ...
Removing python3-cups (1.9.73-0ubuntu2) ...
Removing python3-cupshelpers (1.5.7+20160212-0ubuntu2) ...
Removing python3-defer (1.0.6-2build1) ...
Removing python3-dev (3.5.1-3) ...
Removing unity-scope-openclipart (0.1+13.10.20130723-0ubuntu1) ...
Removing python3-feedparser (5.1.3-3build1) ...
Removing python3-gdbm:amd64 (3.5.1-1) ...
Removing python3-pyatspi (2.18.0+dfsg-3) ...
Removing unity-webapps-common (2.4.17+15.10.20150616-0ubuntu2) ...
Removing python3-greenlet (0.4.9-2fakesync1) ...
Removing python3-guacamole (0.9.2-1) ...
Removing python3-html5lib (0.999-4) ...
Removing python3-httplib2 (0.9.1+dfsg-1) ...
Removing python3-idna (2.0-3) ...
Removing python3-jinja2 (2.8-1) ...
Removing python3-jwt (1.3.0-1) ...
Removing python3-louis (2.6.4-2) ...
Removing unity-scope-devhelp (0.1+14.04.20140328-0ubuntu1) ...
Removing python3-lxml (3.5.0-1build1) ...
Removing rhythmbox-plugins (3.3-1ubuntu7) ...
Removing python3-mako (1.0.3+ds1-1ubuntu1) ...
Removing python3-markupsafe (0.23-2build2) ...
Removing python3-mysqldb (1.3.7-1build2) ...
Removing python3-padme (1.1.1-2) ...
Removing python3-pathtools (0.1.2-2) ...
Removing python3-pexpect (4.0.1-1) ...
Removing python3-pil:amd64 (3.1.2-0ubuntu1) ...
Removing python3-setuptools (20.7.0-1) ...
Removing python3-virtualenv (15.0.1+ds-3) ...
Removing python3-problem-report (2.20.1-0ubuntu2.1) ...
Removing python3-ptyprocess (0.5-1) ...
Removing python3-pyasn1 (0.1.9-1) ...
Removing python3-pycurl (7.43.0-1ubuntu1) ...
Removing python3-pygments (2.1+dfsg-1) ...
Removing python3-pyparsing (2.0.3+dfsg1-1ubuntu0.1) ...
Removing python3-renderpm:amd64 (3.3.0-1) ...
Removing python3-reportlab (3.3.0-1) ...
Removing python3-reportlab-accel:amd64 (3.3.0-1) ...
Removing python3-speechd (0.8.3-1ubuntu3) ...
Removing python3-systemd (231-2build1) ...
Removing python3-tk (3.5.1-1) ...
Removing python3-wcwidth (0.1.6+dfsg1-1) ...
Removing python3-wheel (0.29.0-1) ...
Removing totem-plugins (3.18.1-1ubuntu4) ...
Removing python3-xdg (0.25-4) ...
Removing python3-xkit (0.5.0ubuntu2) ...
Removing python3-xlsxwriter (0.7.3-1) ...
Removing python3-yaml (3.11-3build1) ...
Removing qml-module-io-thp-pyotherside:amd64 (1.4.0-2) ...
Removing rhythmbox-plugin-zeitgeist (3.3-1ubuntu7) ...
Removing rhythmbox (3.3-1ubuntu7) ...
Removing ubuntu-minimal (1.361) ...
Removing ubuntu-system-service (0.3) ...
Removing ufw (0.35-0ubuntu2) ...
Skip stopping firewall: ufw (not enabled)
Removing unattended-upgrades (0.90) ...
Removing unity-scope-calculator (0.1+14.04.20140328-0ubuntu1) ...
Removing unity-scope-chromiumbookmarks (0.1+13.10.20130723-0ubuntu1) ...
Removing unity-scope-colourlovers (0.1+13.10.20130723-0ubuntu1) ...
Removing unity-scope-firefoxbookmarks (0.1+13.10.20130809.1-0ubuntu1) ...
Removing unity-scope-gdrive (0.9+16.04.20151125-0ubuntu1) ...
Removing unity-scope-home (6.8.2+16.04.20160212.1-0ubuntu1) ...
Removing unity-scope-manpages (3.0+14.04.20140324-0ubuntu1) ...
Removing unity-scope-texdoc (0.1+14.04.20140328-0ubuntu1) ...
Removing unity-scope-tomboy (0.1+13.10.20130723-0ubuntu1) ...
Removing unity-scope-virtualbox (0.1+13.10.20130723-0ubuntu1) ...
Removing unity-scope-yelp (0.1+13.10.20130723-0ubuntu1) ...
Removing unity-scope-zotero (0.1+13.10.20130723-0ubuntu1) ...
Removing libgnomevfs2-0:amd64 (1:2.24.4-6.1ubuntu1) ...
Removing libgnomevfs2-common (1:2.24.4-6.1ubuntu1) ...
Removing gconf2 (3.2.6-3ubuntu6) ...
Removing python3-requests (2.9.1-3) ...
Removing python3-urllib3 (1.13.1-2ubuntu0.16.04.1) ...
Removing dh-python (2.20151103ubuntu1.1) ...
Removing update-notifier (3.168) ...
Removing update-notifier-common (3.168) ...
Removing update-manager (1:16.04.3) ...
Removing ubuntu-release-upgrader-gtk (1:16.04.14) ...
Removing update-manager-core (1:16.04.3) ...
dpkg: warning: while removing update-manager-core, directory '/var/lib/update-manager' not empty so not removed
Removing ubuntu-release-upgrader-core (1:16.04.14) ...
Removing python3-distupgrade (1:16.04.14) ...
Removing python3-debian (0.1.27ubuntu2) ...
Removing python3-chardet (2.3.0-2) ...
Removing python3-dbus (1.2.0-3) ...
Removing python3-gi (3.20.0-0ubuntu1) ...
Removing python3-pkg-resources (20.7.0-1) ...
Removing python3-six (1.10.0-3) ...
Removing python3-update-manager (1:16.04.3) ...
Removing lsb-release (9.20160110ubuntu0.2) ...
Removing python3-apt (1.1.0~beta1build1) ...
Removing python3 (3.5.1-3) ...
Processing triggers for bamfdaemon (0.5.3~bzr0+16.04.20160523-0ubuntu1) ...
Rebuilding /usr/share/applications/bamf-2.index...
Processing triggers for desktop-file-utils (0.22-1ubuntu5) ...
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Processing triggers for shared-mime-info (1.5-2ubuntu0.1) ...
Unknown media type in type 'all/all'
Unknown media type in type 'all/allfiles'
Processing triggers for dbus (1.10.6-1ubuntu3) ...
Processing triggers for gnome-icon-theme (3.12.0-1ubuntu3) ...
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Processing triggers for libc-bin (2.23-0ubuntu3) ...
Processing triggers for initramfs-tools (0.122ubuntu8.1) ...
update-initramfs: Generating /boot/initrd.img-4.4.0-28-generic
Processing triggers for doc-base (0.10.7) ...
Processing 3 removed doc-base files...
Registering documents with scrollkeeper...
Selecting previously unselected package kdesudo.
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Why did it remove firefox, docky etc?

@StarOS' answer (now removed) was correct in that by removing python3, you also removed a lot of critical dependencies needed by your system, and a great many other packages.

However, he/she was not correct in recommending you to use Boot-Repair. Instead simply go to the terminal with:

• sudo apt-get install -f (the -f- option is also written --fix-broken which is more explicit), just in case your system was broken in some way. (Go to man apt-get for more info).
• followed by sudo apt-get install python3

As suggested by @DavidFoerster in a comment below:

 $ sudo apt-get install -f ubuntu-minimal ubuntu-desktop

will actually rebuild your desktop, re-install your missing packages, among them python3. This will completely fix your install.

On Debian testing, for the main suite in the amd64 architecture, there are at least 365 packages with direct dependency to the python3 package (listed in the Depends, according to ben query '.depends ~ "python3"' Packages_amd64 | grep -E 'Depends: .* python3 ' | wc -l), and given Canonical plans to make python 3 the default, it wouldn't be surprise that there are a bunch of packages that depends on it. That's why python3 package is important, but notice how it doesn't cripple your system... you can actually recover from it if you install the meta-package that pulled all the window/desktop manager packages, namely ubuntu-desktop.

Now, why did apt decides to remove other packages because you removed python3? Because apt has a dependency resolver that makes sure that after every operation no packages has unsatisfied dependencies, and when it can't find a solution that doesn't leave a "broken" dependency, it refuses to remove it. APT did warn you about all the packages being removed which were listed in "The following packages will be REMOVED" list. You accepted it and then it proceeded to remove them.