Have you met any fat people?

I mean that in all seriousness: Talk to some fat people. If you are close enough to them, ask them why they are fat. The reasons they give will generally boil down to "I couldn't possibly meet the standards of our society, so why bother? It's easier to just eat whatever I want and never exercise."

People with healthy body image concepts are sometimes "overweight" (in BMI, etc.), but they are never really fat---they don't ever reach full morbid obesity. This is because they are concerned about their health, not how they appear to others.