[fulafel]

For many people, once you keep a certain weight for a while, your body will try pretty hard to keep you there. This means that your metabolism will slow down if you start doing the straightforward small calorie deficit.

Hence to lose weight you need to keep your calorie deficit at high levels and feel significantly hungry for a very long time.

This comes with a risk of yoyo-ing, which is much worse for you than keeping the same weight since it will put high stress on your insulin/lipid metabolism that causes insulin resistance and type II diabetes. Indeed most overweight people have gone through cycles of yoyo-ing and ended up heavier than what they started with.

TL;DR: if you think obese people are just unusually lazy or weak willed, you aren't getting the picture.