Guys why do you just stare at a girl but not speak?

Like why would a guy just look at you everyday, but his friends will look at you also but they speak. He is most outgoing and talks to everyone else but me just looks. Even when we are close he keeps looking in my eyes but doesn't say anything, when he walks he comes over so close he is about to fall over my knees when i am sitting down and keeps looking at me face all the time.

If you speak to everyone else but why not me. Don't say he is shy because when he sees' me he thinks he all cool and talks non-stop. All he does is give me a blank look or smile, which I smiled back at him before. I said hi don't think he heard me, but why look at me, its like you haven't got a mouth or anything,