Please help, Why is he doing this to me?

There is a guy at work I find attractive. The problem is that he acts super weird. One day at work he is super fun and flirty with me and we joke around all day and it feels wonderful. The next day or two he seems to avoid me and it feels like we are strangers. WTF is happening and what do i do?
Please help, Why is he doing this to me?
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