Why do guys act like they like you and then just ignore you?

I seriously am starting to get annoyed by him. He acts as if he's seriously interested in me one day and then just completely ignores me the other day. He won't talk to me, only stare and look stupid. Okay, we don't even talk a lot at all, but he doesn't even greet me when I walk in. We work... Show More

Sorry, I know not all guys are like this and there are most definitely girls out there who do the same, I'm just really frustrated.

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