I put genetics, psychology, environment, and biology. Here's why:
Genetics: I inherited my dad's bone structure and fat distribution, which made me genetically prone to being bigger in unfavorable areas. Any time I have any extra weight, it shows in my face and my stomach.
Psychology: This one plays a big role for me. I was a fat kid in middle school and got tormented for it, so I had eating disorders from the ages of 15 through 18. I didn't completely stop my eating disordered behavior until I moved down to Florida to be with my girlfriend, who worried about me so much and made me eat. I decided that I had been depriving myself of yummy foods for so long that I went on a big ol' binge and gained about fifty pounds in around six months. Then, my weight leveled out, and while I stopped binging, I still ate more than I needed to - AND I was no longer physically active. When I was thin, I lived in DC and walked everywhere, plus I was very busy and involved with so many things that I conveniently "forgot" to eat. My relationship with food has never been healthy until now... it has always been attached to feelings of guilt mixed with feelings of pleasure and satisfaction.
Environment: I packed on all the weight when I moved in with my girlfriend, who loooooves to cook. She's Puerto Rican, so food is LOVE, and if I didn't want to eat something she made, she'd pout at me, get sad, and say, "What? You don't love me?" And then, even if I was full, she'd put more food on my plate because she looooooves to FEED ME! I think her mother does the same thing to her father... and he's HUGE, too.
Plus, I heard for some very peculiar reason that people who move down to Florida from parts further north gain weight. I'm not sure why, though.
Biology: My biological make-up doesn't like being skinny. Some people truly are meant to be skinny, and I'm just not one of them. Yes, I was skinny for 4 years, but unnaturally so. As soon as I started eating again (and eating like a normal person, which really isn't healthy at all), I gained all the weight back AND THEN SOME!