This is one of those popular common misconceptions that men sometimes have. It probably comes from some sort of collective male paranoia (women are out to get us.) When I was in college, it was explained to me that, no, women are not all friends, just that women are less likely to be overt in their distaste or dislike for others. I don't pretend to understand why this is, but since then I've just sort of come to accept it as a fact of life.