As a male myself, I might be able to give you some insight... however, I'm not sure how old you are (you're 19 right?), so my advice may not be applicable...
Chances are, something you did or said to the boy implied that you liked him. Happens to me all the time, for example, one time there was this Muslim girl I really liked (it turned out that she was an evil false-goddess bent on killing me, but that's another story), that rejected me to prom because she said she didn't date. The next week, I asked her why she didn't date, and she said that she didn't want to date until college (I found out later that she was lying. Her real reason was that she wasn't allowed to date anyone who wasn't a Muslim from Pakistan). Well gee, she was a senior, and I was going to college next year also, so what do you think that told me? Remember, a boy who's head over heels for a girl will take ANYTHING that even HINTS that you like him (or even that hints that you don't hate him, depending on how bad the crush is) as a sign that you definitely like him and are willing to go out with him. For example, if he says "you look pretty today," and you say "aww, thank you," in a sweet, gentle voice (like most girls do when being complimented), he will take it as a sign that he has a shot with you.
If this boy is anything like me, his whole blissful "in love" state will have been completely shattered when you suddenly told him that you have no interest in dating him, and he'll have started going paranoid about how you planned out his feelings from the very beginning so you could sadisticly watch him suffer in the end (I've thought this about girls, actually. In fact, I'm pretty sure the ones I've went through really did plan it all out.). If this guy's the living and completely accurate clone of me, he may have even begun hearing voices in his head telling him that you want to kill him (in a boy's mind, or at least mind, there is a strong subconcious connection between rejection and death).
That give you any information you need to know?
EDIT: Also, whatever you do, please, please, PLEASE DO NOT tell the boy that you never want to talk to him again and to just forget about you. That scars the boy for life (trust me, it's happened to me before), assuming that he doesn't commit suicide, which weaker-willed boys might in the event that the girl of their dreams just shoved them away like that.