Why don't girls seem interested in me? ?

Well before I start, I'm just curious about this it doesn't really concern me that much. Not a single girl has ever showed interest in me. I'm usually the "friend" type. The only reason I can think why girls aren't interested is because i'm shy sometimes. I'm 19 and never had a girlfriend before or kissed a girl. (I'm more concentrated on school anyways. No i'm not a nerd). So does anyone have any ideas of why females don't like me?