What causes increased appetite? I get more hungry than people do and not full from eating a whole meal?

I feel like my brain is not getting the signal is needs to stop eating. The body just feels like it needs more food even though I ate plenty of it. It thinks its hungry all the time, and I don't know if it's because of a medical condition, or my body weight or whatever. I've spoken to my doctor and he doesn't find any problem, but I'm STILL feeling these hunger symptoms and its really bad.It really bothers me and I've been trying to make changes to my diet, and getting excercise. Even with a healthy lifestyle, my appetite is still increasing and it irritates me.