Why do girls hate each other?

A male friend of mine once told me "Women know women and they don't like them".

Women pretty much can read other women and not trust them.

1.) It usually boils down to jealousy. 2.) We don't let anything go. We hold grudges and we'll remember everything.

Women are very social creatures by nature. In their objective of "attracting" a man, they mostly use their social network to attain their goal. Women essentially "ask" the group if they can date a man. If the group agrees, then there are no hard feelings. Typically though, a few girls will want the same guy. That's when the proverbial gloves come off. They resort to starting sometimes mean rumors in order to marginalize the competition. It then becomes a war of escalation.