For me it was a history of mental abuse from my peers, and an inability to cope with my emotions. I ate rather than dealing with the problems that were around me. I ate to cope with everything from sadness to depression. I'd done the yo-yo dieting thing since I was 12. I'd tried going vegetarian, I tried cutting out carbs, I tried Weight Watchers, TOPS, Jenny Craig, all before I ever hit 21.
Well, as you can guess, I kept going up and up and up, always gaining about twice what I'd lost. But I managed to hit and stay at a maximum of around 300-320 lbs, thanks to a stupidly high amount of exercise, since I was 'active', even when I was fat. I could run up and down a flight of stairs without getting winded, and I was starting to work around my emotional issues, and things were looking good!
Then at 23 I fell and blew the cartilage out of both knees, after which I was diagnosed with PCOS and had to have surgery, coupled with losing my father to cancer. Between the emotional and physical issues, I just went immobile. I stopped being able to dance and run the way I used to, and I just quit caring about anything, so I started trying to eat myself to death, despite having a wonderfully supportive husband and a halfway decent life regardless of the troubles I was having at the time.
I ended up almost 500 lbs at my heaviest. It wasn't a pretty journey, but I'm determined not to go back to where I was.