Since I've been overweight most of my life, I've often thought about what factors cause a person to be fat. Is it biology? Psychology? Both? The difference between the way I ponder this point and the way it is generally bandied about in the media and by many people (both those who are thin and those who are fat but trying to lose weight) is that I do not concern myself with this issue in order to assign "blame" or "responsibility". I'm not looking to find out why in order to let someone off the hook or to hang them on one. I simply want to understand the factors that go into weight problems so that solutions, when they are desired, can be found.
One of the things I realized lately is that the accusations related to being fat are based in a bias which we supposedly have overcome. If the person's weight issue is deemed to be a physical issue, then we "forgive" them because it's not their fault. If the person's fatness is the consequence of a psychological issue, then we say it is their fault because they are "weak-willed", eat emotionally, or are lazy, or piggish. In essence, if overeating is a mental health issue, we feel they should just be able to overcome it alone by sheer force of desire.
Psychology, in my opinion, always plays a role in eating. We have a psychological relationship with everything because we are human and have feelings about everything. Unless you're in a coma (and maybe even then... I've never been in one so I don't know), your limbic system is going to be engaged in regards to food. To deny this is foolhardy. The only issue at play is whether or not you have a psychological relationship with food which results in physiologically undesirable results (a lack of health) or desirable ones (robust health).
It's important to keep in mind that each individual's biology and psychology are unique when it comes to food. A person with Type 1 diabetes can have a perfectly "normal" psychological relationship with food and eat within caloric limits and still have a degradation in health. A person can have a horrendously bad relationship with food like compulsive eating and not suffer any ill effects if that compulsion leads them to eat say, a dozen oranges everyday. So, the situation is never simple.
At any rate, one thing that I have realized is that much of the disgust and judgment of fat people is based in the idea that psychological issues with food which lead to being overweight are to be regarded with disdain, anger, and condemnation. If we consider that any weight gain as a result of a psychological issue is a mental health problem, then it would seem that the underlying idea is that it is acceptable to blame someone for a mental health disorder, but not a physical health problem.
Some people would deny that the psychological factors which compel people to overeat can be classified as true "mental health" issues, but this is merely an attempt to justify fat hating and the underlying processes that drive it. Any behavior which causes you to behave destructively toward yourself (or others) is a mental health problem. Any emotional problems which drive you to do things which promote poor health are mental health problems. Mental health problems don't only cover the extremes. They cover a wide variety of quality of life issues, including emotional issues which drive people to food for comfort.
The irony is that fat people, who are often the victims of abuse, often tacitly buy into this prejudice as well. When they attempt to offload their weight problems onto metabolic disorders, medication, etc., they are saying, 'my problem with weight is a medical one, not a psychological one, so please don't blame me the way we blame fat people who "can't control themselves."'
The attitudes of fattists, and even fat people who apologize or nervously offer medical excuses for their weight, can be boiled down to a mental health disorder prejudice. We can't judge and blame people with medical conditions that lead to weight gain because that's like blaming someone for developing cancer. Apparently though, we can blame people with a psychological issue for their inability to simply "get over" the problem. The anger directed at fat people is akin to "blaming" a chronically depressed person for simply not "manning up" and "cheering up." In other words, a person with a weight problem, when that problem is in whole or in part motivated by psychological issues, simply cannot "eat less" and "exercise more" anymore than someone with any other mental health problem can simply decide to be well by "snapping out of it".