Girls aren't able to regulate their overwhelming emotions well, so they get bogged down and are told things like "pressure valve release" and such are an escape. So they cut, get that pain euphoria (helped by their unacknowledged masochism) and then are conditioned into a vicious cycle.

Its the same with all their other "conditions" like anoxeria/bulimia or bleaching their hair to baldness. A bunch of girls told them it "makes them feel better" and then they turned to it in a bad moment once, only to then get addicted.