It bothers me that implicit in this thread is that fat equals lazy, stupid, mentally ill or some combination.
Though this may be true in some cases, it's not in all of them. We talk about racism and ageism, but what about fatism? I know very many people that look thin and healthy who are just as lazy and slobby as people that are heavy. Bodies are different. People are different. Frame sizes are different. You absolutely cannot look at two people, especially if they are wearing clothing, and determine who is healthier based on their body size.
For me, pregnancy and childbirth, quitting smoking, and medication were all things that triggered metabolic changes such that I gained weight while maintaining the same level of activity and the same diet. My weight was about 130-140 for many years. At my heaviest, I was about 240. Once you get caught with that much extra weight, it's hard to lose because you actively have to eat less than your body wants and have more physical activity. Right now, because I am in the middle of a weight loss program, I eat better and am more active than I have ever been in my life. It is not easy, and having discipline to stay with it is hard. The damage that has already been done is much harder to correct than maintaining any weight, fat or thin.
There are a lot of cultural factors in the US at least that contribute to obesity-- working long hours and easily available fast food are one trap that many people get caught in. We are so exhausted from working that we either don't eat or just pick up something at a fast food joint. Of course, one meal from one of those places can be more calories, fat, cholesterol and sodium than you should have in a day. Yet a few hours later, you feel hungry.
My point is, it's not fair to assume that people that are overweight are somehow lesser human beings because of their weight. In most cases, it is simply not true.