Why do some guys stare but never initiate conversation?

jstcurious
I see this guy on campus. We don't know each other. The first time I noticed him we were passing each other and he was holding eye contact with me until we passed each other.I didn't think much of it.The next time I was sitting and he was coming over to greet the guy next to me, but was looking at me the whole time.Another day I came into the classroom and he was packing up to leave.I sat down and when he looked at me I smiled and looked away.Out of the corner of my eye I could see that he was still staring.On two occasions he's gave me the "up and down" (i guess) he would stare at my shoes or my lower body, look up to my face, and then look away. Today we walked by each other and as always he made eye contact but looked down once we kinda passed by. I just smiled, I wanted to say something because I figured he never will, but I froze up.He always just has a relaxed facial expression.I just don't know how to read him.
Why do some guys stare but never initiate conversation?
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