People who are seriously obese are that way for a broad range of reasons. Some of these reasons are medical, some genetic, and some psychological, but all of these are generally ignored in favor of "you're just lazy".
How do I know this? When I was eight, my thyroid stopped working. Despite a family history of thyroid problems, my doctor didn't even deign to have me tested. He said that hypothyroidism was "overdiagnosed," and for the next four years I was subjected to an onslaught of dietitians and other specialists who treated me like I just wasn't trying hard enough.
Understand, my mother was counting the grapes she put in my lunch. No healthy amount of dieting could have allowed me to lose weight; I was gaining it because my body didn't work properly. That didn't matter though. Even after I was diagnosed, I never got an apology from anyone involved. They felt they were right to treat me the way they did; after all, I was fat, so I was clearly doing something wrong.
