Why would he do this - is this all in my head?

At work I have this weird thing with a coworker who I like but I don't think likes me back as he has never made a move in any way. We have a general weird tension which I'm confused as to whether it's all in my head because I like him or if it's actually mutual... But sometimes when I enter the room he'll look away like you know when someone pretends not to see you, while everyone else will greet me etc. Does this mean anything or am I reading too much into it? Guys what would be the reason you would do this?