I think GENERALLY girls tend to have a problem with one another more often then guys do. Women like to talk, share opinions, and are more interested in other peoples business (again, GENERALLY) then men are.
Ex. When one of my girlfriends breaks up with her boyfriend we get together and talk about it, and cry, and share stories etc. We go over every detail.
When my fiancé comes home and tells me that one of his friends broke up with their girlfriend, he has no idea why, what happened, or how his friend feels about it.
This talking, and strong opinion, can lead to drama and gossip if you are not careful.
Also, when girls are insecure they are easily threatened by other women. I'm sure men go through it too, but they probably express it in different ways.
Girls for sure
I think girls hate other girls for being prettier than them.
I just hate everyone equally.