There are a number of causes however the only 1 have seen in person is depression/bulimia they go hand in hand. My friend had her "flaws" pointed out and at the time she was clinically depressed so her self esteem was not the best. This caused her to become bulimic and i did some research before i tried to help her and being bulimic can cause anorexia. That's one i also believe sometimes your just born with it but don't take my word for it.
There are many factors which are correlated with anorexia. One is our society's definition of thin as beautiful. The aneroexics need to control vis a vis an over involved and controlling mother. Food is seen as nurturing that which is hated, the self. There are many papers written on the psychodynamics of the fractured sense of self in the anorexic. There is no one cause for anorexia.
There are many causes, low self esteem, a desire to appear as the waifs in fashion magazines, bullying,problems at home. can be contributing factors. personally I like girls who are w/in they're proper ht/wt/chart parameters. Not too skinny not too heavy.