He keeps an eye on me, but doesn't come and talk to me, why?

I always catch him staring, or glancing at me. But he almost never talks to me. At my middle school, there is recess for 30 minutes because the high schoolers have an hour of lunch. Anyways, he sometimes migrates around me, meaning that I can always see him or he can always see me. He knows that I want to talk to him. But the only time he talks to me is in P. E. and that's only side comments about the class. Although, on Friday, he left a smiley face on the paper he graded and it was mine. I graded his. I want to talk to him and everything, but I can't find any convo starters so it ends up to being a greeting but 10x more awkward. He is also really nice, just to put that into account. Anyways, why doesn't he talk to me, but is keeping an eye on me?
He keeps an eye on me, but doesn't come and talk to me, why?
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