Why does this guy talk to others girls but not me?

This guy that I like at my school always stares at me but NEVER TALKS to me. When I catch him staring, he'll try to hold eye contact with me (he stares from across the room). My friends always tell me he's staring. He knows I like him, but sometimes I don't show it. I have no idea why he stares so much. One thing that concerns me is that he talks to all these other girls (not in front of me) & he flirts with them, but he can't talk to me! I worked beside him before & he never talked to me, he'll tease me when we're with another co-worker but alone he's quiet. Why can he talk with others girls & not me?