Why does apt-get sometimes asks for confirmation?

The confirmation dialog will not only appear when you would install packages you didn't ask for, but also in the following cases:

• Installing packages that you didn't specifically want to install
• Removal of essential packages
• Changing a held package _{(a held package is one you want to keep in the version currently installed)}

You can override it with one of the following switches:

• -y
• -yes
• --assume-yes

Which acts as if you answered yes to all those questions. However, prompts for essential packages will still be displayed for safety reasons.

Essential packages:

Essential is defined as the minimal set of functionality that must be available and usable on the system at all times, even when packages are in an unconfigured (but unpacked) state.

Thanks to @Liori for pointing that out.

If it needs to install more than the packages you explicitly asked for, it'll ask for permission, else it won't, i believe. If its an annoyance, you can use the -y switch to automatically assume yes.