And I say that as not someone who is a fat shamer, but as someone whose love of food allowed myself to get up to 280 before realizing that I really needed to regulate my diet.
Sure some have medical problems but most I know, is just flat out neglect.
And I'm sure some people have psychological issues which make them overeat, but I would say that the vast majority of people are just lazy about exercise and losing weight. That's why there are so many "quick & easy" diet fads & pills on the market nowadays. People want the easy way out without realizing that you can't just take a few pills and lose weight without changing your diet.