There are many theories. For a particular person, all of them or only a few factors come together to finally produce the problem.
Biological Factors:
Possible abnormality in the noradrenergic transmission system (Low levels of norepinephrine in the cerebrospinal fluid of patients with AN). Recent studies have shown the likely decrease in a protein called Leptin, which comes from adipocytes (fat cells of the body). Currently there are some theories in research that associate its origin with an infectious problem. Temperament, which is partly determined by genetic factors, causes some people to be more vulnerable than others to developing eating disorders. In addition, once a person has started with periods of starvation, binge eating or purging, these behaviors can alter brain chemistry and prolong the problem.
Psychological Factors:
People with eating disorders tend to be perfectionists. They usually have a legitimate "courage", but since they always seek approval and fear criticism, they do not know how to express their anger in a healthy way. They turn to themselves causing starvation or binges.