Why would a guy act cold towards a girl he likes?

My coworker always acted like he had some sort of crush on me. He stared at me a lot, always initiated conversation, acted awkward around me, and kept trying to help me with things. He's 22 and never had a gf before (so I've heard). I really like him too, but he does not know that.

I'm really shy so I come off as cold and distant when I don't mean to. I think he may have picked up on this and thought I was being mean to him? I don't know.. but now he started acting all cold and distant and weird around me for no apparent reason. He always makes conversation with me thoughout the day and is really nice, but then is rude other times. I will say hi to him and he will ignore me with no expression on his face and just walk away. Other times he will not attempt conversation and we just sit in silence and he looks mad or something. He is still awkward. I don't get why he acts like this one minute and then twenty minutes later he is really nice and makes conversation? I don't like this
Why would a guy act cold towards a girl he likes?
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