Eating disorders are generally developed from psychological issues, such as depression, distorted body image and sometimes a result of some sort of abuse. Control is a key factor in anorexia....having control over what you put into your body sometimes is the only control the person feels they have over their lives...sometimes this springs from a strong desire for perfection. It really depends on the individual.

I suggest it's from the thought they don't like their body and the desire to look extremely thin.
So they go to extreme lengths to lose weight.