Why women need more than one girlfriend...
So they have somebody to talk to about their other girlfriend when they are currently pissing them off.

We all do it, even as adults when it's not spiteful or malicious. It goes something like, "I love her, you know that, BUT sometimes..." And then you and your friend can vent until next week when they are talking to each other about YOU. It's a vicious cycle really, a catch-22.

PS: Remember that 80s board game, Girl Talk? I used to play it at my friends house.