Yosemite 10.10.3 asks for google password every week with 2 factor authentication
As far as I know, Google still considers some apps (including Apple's) to be less secure, as they call them. As a solution to be safe, they can force you to reauthenticate from time to time.

My solution, that I'm happy with so far, is to use their App Passwords for every single app that is less secure and does this annoying reauthentication step.