There’s a thing called set-point weight where your body will fight against weight loss so with the massive amount of weight loss needed it’s important in the maintenance of the weight loss. It’s believed to be about 7 years before the set-point weight “resets”; so it would take losing the weight and maintaining the loss for about 7 years before the brain recognizes the new weight as the set-point weight. (Simplified Version)

This is why diets are very difficult to maintain. There’s also the added point that once a person is overweight, it’s easier for that person to gain weight than someone who’s never been overweight due to more fat cells present.

There’s hormonal factors as well that cause a person to overeat. This isn’t a matter of “willpower.” These people need surgery as the best hope of both losing and maintaining the weight loss.