A male coworker and I have been texting for about 3 years. I told him I had feelings for him, and he told me that he can't give me what I want. So, obviously I accepted what he said and we have stayed friends. The issue is he is extremely jealous and possessive if I talk to other males, he will slap his desk, glare at me, and sometimes even say something about it. He will watch me all the time, and one day he will be nice and the next he will be distant. Can someone please explain what is going on in his head? Have other people behaved like this, and if so why?

Why can't he give you what you want? Is he married?

This is why workplace relationships often don't work out. Can you ignore him, even do a No Contact thing? How heavily are you involved with him as a coworker?

Sounds like a bit of the "If I can't have you, no one can" mentality. He may not want to lose out on your attention and views other men as competition. I'd steer clear of him. His Jekyll and Hyde behavior doesn't sound very friendly. You may have to decide whether this friendship has run its course and if continuing it is in your best interest, both personally and professionally.

He has personality issues and maybe mental problems I don't know but this is NOT normal or good behavior and you better be cautious and protect yourself and keep this nut at a safe distance and out of your business.