Why does he act jealous if he doesn't want me?

We are both 21, coworkers. So he and I have hung out alone a few times (we never called them 'dates', never had sex or even kissed) He used to text me every night but recently he completely stopped contacting me. And he acts standoff-ish at work so I have just been taking my cues from him. I'm slightly hurt but I have not contacted him, my pride is damn near steel.

Now whenever we are at work he is ONLY interested in me if I am speaking to another guy. Ever! He does not talk to me when we are alone. If I'm laughing with a guy he will either go cold as ICE on everyone or completely interrupt our conversation and dominate it with dumb comments or questions about me and my activities. Huh? Maybe you'd know that if you'd speak to me once in a while! He seems jealous, but that doesn't make sense to me since he knows he is the only coworker with my number and he could get in touch with me any time! He will also come around when I'm standing with a group of people, stare at me, but only speak to everyone else? We have already been out alone together many times, so the fact that he is shy can't be the problem, right? A coworker even asked me, "Is Daniel in love with you? He stares at you kind of... a lot." But I'm too annoyed to care at this point.

Please explain this confusing man.
Why does he act jealous if he doesn't want me?
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