Guys always assume that I'm not interested and that I don't really like them, why?

okay, I'm the quiet, reserved, slightly shy type. I don't have many friend and even most of the time at school I am by myself. I have no problem with that. I can talk to people and when we get acquainted I can be really funny. Also, I am never afraid to speak my mind (but not in a I am always right kind of way) and actually I enjoy a good debate. I guess though I am quiet I am usually friendly to everyone. Recently I've come to the realization that guys who were really interested in me assumed I wasn't and that I didn't like them. Is it because I can act aloof sometimes? or because I have this independent, I am good by myself vibe ? I mean, I am not a cold blanket, it's true that it takes me a lot of time to notice that a guy is interested in me, but when I do and that I am I try to be more friendly around them and with them. Besides I can flirt a lot and usually it's in a teasing way..

But anyway, I am really confused as to why they would go as far as thinking that I am not really interested in a relationship ! (they always first assume that I already have a boyfriend but since I never mention it I guess they end up knowing that I don't have anyone...) What makes them think so? What can I do in the future to avoid this?