He stares at me but...

So this guy I know, whenever he sees me, he stares at me. And I saw him twice in passing today, the first time, we just had an awkward staring contest. The second time, I walked by and said hello but he didn't say anything back. Why bother staring at me but not saying hi back?

We have a class together and he's talked to me during it. He's put his arm on the back of my chair and he asks me about my life when I sit next to him and tries to get to know me. I'm just confused. What gives?

Someone? Anyone?