Ken Jacobsen is a counselor at the University of Northern Iowa's counseling center. He says it begins with society at large, and filters down to individual groups. Teenagers do the same thing. Young girls routinely starve themselves to achieve a certain look. Boys have become fussier about their hair and physical appearance than most girls typically become.
Children steal, and even commit violent acts, to have the right tennis shoes.
Nobody wants to be different, to be out. And in my experience, those standards are getting increasingly narrow. Further, even if you do fit into one of the groups, you've got some pretty rigid expectations to live up to. If you are perceived to have wavered from some of those expectations, you may find yourself ostracized. And at an age when self has yet to be developed, that can be devastating, even deadly.