sudo doesn't ask for a password, even after multiples reboots
Solution 1:
Use visudo to edit your sudoers file and look for NOPASSWD: - that's the directive disabling the user password prompt (you NEVER have to enter the password of the target user, i.e. the root password). Simply removing that directive (including the colon at the end) should require you to re-enter your password to use sudo (unless you used it recently, then it's still cached, you can clear this using sudo -k)