Depression is extremely common among patients with severe eating disorders. Often this depression can lead to suicide. You'll find many patients in eating disorder programs being prescribed anti-depressants to deal with this.
It is true that many patients become extremely depressed. One author of a best-selling book on eating disorders calls this "Confirmed Negativity Condition, or CNC. She believes this underlying negativity toward self is the underlying cause of the eating disorder. If you ask any eating disorder counsellor, they'll tell you that depression places a big role in precipitating anorexia nervosa or bulimia nervosa.
It is of key importance here to realize that the therapist usually meets the patient for the first time only after she is in the advanced stages of her illness. The patient by then will have been struggling with her eating problem for many months, and may already be in an advanced stage of malnutrition. What most therapists do not realize is that if you talk to the parents of anorexic patients, most will say their child seemed pretty normal for most of their life, up until this started. To assume that the depression is a precursor to disordered eating is extremely speculative.
What is far more probable is that severe depression develops as a result of the patient's life basically falling apart due to the eating problem. She may have been a perfectly happy person up until this started. Now her parents don't trust her, they argue with her at mealtimes, she's not allowed to play sports anymore, she had to give up all her favorite activities, her friends think she's weird and have distanced themselves from her, the nutritionist keeps telling her to eat foods she can't tolerate, she has to stick to a rigid mealplan, nobody listens to her when she complains of stomach aches, she's so constipated and bloated that she feels fat and ugly, she's treated like an insane person, and her social life is non-existent. It's no wonder she's feeling depressed, and if you add to these the prospects of hospitalization and possibly being put on a psychiatric ward, suicide begins to seem like a better alternative. Or log onto a pro-anorexia website and at least find someone else who's going through the same thing. There she doesn't get criticized for being so thin, in fact she's accepted and encouraged in her behavior.
The therapist usually will see the patient for a few minutes per week, either as an outpatient or in hospital. In these few minutes they will make their psychiatric assessments of the patient and possibly reach many conclusions about her emotional state and even her family situation. Because they are only looking for contributing factors, they turn a blind eye to all the healthy aspects of her family relationships and focus on anything that could have possibly triggered this terrible depression and self-hatred. Just about any stress that's occurred in the family is viewed as a contributing factor.
Often the depression is believed to be a result of low self-esteem, and many therapists interpret the dysfunctional eating as a cry for attention or a means of punishing herself. Many therapists believe that anorexic patients are trying to starve themselves to death. They assume that anyone who could do such a thing must really hate themselves.
You can see that all these theories are based on the assumption that the patient's digestive tract is in perfect health, and absorbing all the nutrients properly, and that there is no nausea, gastrointestinal distress or any other physical disorder. This despite there being few if any tests to determine if this is true.
It is understandable that an anorexic patient would become extremely depressed. If there is an underlying physical illness found to be causing the anorexia, then it makes it far easier for all caregivers to feel compassion and empathy for the sufferer. Once the physical disorder is corrected, recovery can begin both physically and emotionally.