Why does my crush get close to me then pull away?

I like a guy from work and for quite a few months we were really close we talked all the time and hung out. Then he went on vacation and when he came back it was like a switch turned off we were back at the beginning again. Now it seems when we get close to where we were and start laughing and talking the next day he's back in his shell all over again. I took it as he's just not interested in me like that but when I try to get close to anyone else like listen to people talk he acts all crazy and tries to distract me by making me laugh. I don't understand what's going on?
Why does my crush get close to me then pull away?
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