I think Marina largely nailed this one.
Beyond that, I think we kind of condition each other to do this (although I can’t really explain why that happens). Girls reinforce other girls’ criticisms of each other by agreeing, laughing, etc.
I started at a new school with a relatively small, tight-knit sub-community last September and have been really fascinated to see the social dynamics in play. I think for whatever reason, women really bond with each other by discussing other people, and these people are disproportionately other women, maybe because women spend more time with women, or women identify more strongly with other women.
The majority of conversations between girls tend to be about each others’ behavior—sometimes critiquing, sometimes analyzing, sometimes judging, sometimes trying to understand, sometimes just complaining. And I think one thing that’s really crucial to understand is that we can criticize each other pretty harshly, and still absolutely, unequivocally love and adore each other. I don’t agree with all the decisions my friends make and I don’t necessarily like everything about them, but I still love them, or else we wouldn’t be friends.