Comment by Macha

The prompt used to show up everytime you log in. If you use multiple computers, an authenticator or multiple accounts, that is quite frequently.

Since YT comments are now G+ comments, I believe it now shows up every time you try to interact with anything, such as like/favourite/comment etc.

I turned my YT account into a G+ page when the option became available as a way to get rid of the prompts without using my real name on YT.