One of my favorite aunts was really overweight. I guess she would have been called obese if you go by the definition. Anyway, she had been that way for about 15 years. There was no medical reason why she gained all the weight. She started putting it on gradually at first, and then over the years it got to be a problem that she couldn't control.
She tried a lot of those lose-weight-fast fad diets over the years. From time to time, she would get motivated and start doing exercises to lose weight, and there were times when she seemed to be headed in the right direction, but she always abandoned her diets and exercise routines, and she would put on even more weight.
She was so frustrated and depressed until her doctor suggested she consider surgery. I know surgery shouldn't be your first choice, but it worked for my aunt. She is no longer overweight and she is so much happier with herself.