Why would a guy friend get jealous seeing you talk to other guys?

He keeps staring with this serious expression every time I talk to my guy friends one on one and when they flirt with me as well.
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He has a girlfriend and he has expressed this jealousy many times even verbally and it's not just this, he keeps staring at me a lot when I'm not looking and quickly looks away when I catch him. Sometimes he doesn't care that I know he's staring. He also tries to make me jealous sometimes. His behavior seems very obsessive with me. He also tried to kiss me once and got really close to me in front of everyone.
Why would a guy friend get jealous seeing you talk to other guys?
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