Why am I always so hungry?

I'm about 5'11", 140 lbs, and 21 years old. I literally eat all day, and whenever I have meals I eat a ton, but I'm still always hungry to the point of feeling like I'll get sick if I don't eat. When I go to Taco Bell I can easily eat six or seven burritos and then eat more when I get home. Even when I eat healthy food I'm hungry; for supper tonight we had grilled chicken, rice with lemon juice and some herb or something, and salad. That was about half an hour ago, and I'm already starving. We had ham, red skin mashed potatoes, pineapple, and green beans for lunch yesterday, and I was still extremely hungry afterward. Anyone have any idea why I eat so much?