What is up with my coworker crush? Am I doing something wrong?

Anonymous
I starting talking to this coworker a few months ago. We seemed to have a lot in common and attracted to each other. He knows I'm the quiet type of girl and don't really talk to a lot of people, but it doesn't seem to bother him. He'd do little things for me at work like help me out with customers and fill in for me on my break. During work I'll catch him staring at me while helping customers or walking by and looking at me. All the people in his area stare at me too. A coworker that he went on a couple dates with stares at me too a lot, they are still friends and I usually avoid them when they are talking. A lot of times, he parks right next to me. I wouldn't really consider us "just friends' because we've never hung out with each other or in a group, although I just got invited to a group basketball game but I declined, also we're friends on Facebook but that doesn't mean anything. I don't make it obvious I like him, in fact, I'll ignore him sometimes, only because sometimes he acts really serious and unfriendly when I say hi to him sometimes. So just recently I started doing the same. I'll ignore him in work meetings or when I'm walking right past him. Sometimes he'll walk by and say hi, I'll just say hi with no smile and keep doing what I'm doing. I'm ONLY doing this because he's been doing the same. Should I be ignoring him? I don't want to seem desperate, but why does he act like a jerk when I'm nice and nice or interested when I ignore him? If we're only meant to be "just friends" why does he act so weird? Why is he so nice to everyone else, but with me he acts so freaking erratic?
What is up with my coworker crush? Am I doing something wrong?
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