I might be able to offer some insight in this. I'm fat, not really morbidly obese but hardly just a litltle overweight. The reasons can be multiple, sometimes its simple depression where food might be a convenient escape. I've personally always been very positive and happy, but this might be because of my food habits rather than the opposite. The moments when I'm truly sully is when I don't eat according to my habits.

For most however I suspect it is simply bad habits, perhaps mixed with other outlying factors such as pressure, mental health or ignorance. My family was never very good or experimentative with food, and while most was probably not unhealthy, it certainly wasn't the staple of great food. However, I didn't gain most of my weight until after I moved out(at the age 19) when I started living on my own. Convenience and laziness won out most of the time, and I simply didn't trust myself to create filling and interesting meals. You pick up some bad habits, for example a good evening is never complete without some soda + snacks.

You keep putting things off, not really giving it much thought as the weight goes up. At certain points you tell yourself you've gone too far and are going to straighten up, but the hurdle is quite bigger than what most think. You tell yourself you won't eat as much, and you will exercise, and you might do so for a few months, hell perhaps even the better part of a year! But eventually you fall back down into old habits, as perhaps you've never really wanted to give them up to begin with. Here is where the biggest hurdle lie, you need to really want to change your habits and more importantly you need to find new habits that works for you. This is an active process that will take time, and you need to actively work at it every day.