I eat even though i'm not hungry, why?

well the tittle says it all i just seem to eat when i'm not hungry, such as now every night i eat a small chi-cargo town pizza, about 4-5 inches, and eat chocolate all day, i am 15 years old, about 4 months until i am 16, now i am not over weight in fact i am completely the opposite, i have a very fast metabolism. why is this?