Why are some men like this?

I have this coworker and he has been teasing me on and off for 2 years now. A few months ago he became really touchy and not liking me talking to certain men. Me and him was talking one day and she just stared at me without looking away until one minute later. He doesn’t like when I don’t talk to him for a whole day so he will ask me for help with doing something. He have shown signs of being interested in me but he won’t make a move. Why are some men like this?
Why are some men like this?
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