Guy's Behavior

Hé has a girlfriend so why is he chasing me and being desperate for my attention?

He has girlfriend but he's been chasing me for over two years now. I keep ignoring him but he won't stop. He keeps staring at me all the time mostly when I'm not looking and quickly looks away if I catch him. He's told all his friends about me and they tease him whenever I'm around. He also often talks to his friends about me when I'm around them and then both he and his friends stare at me.

He's told our mutual friend that he doesn't want a relationship with me but would like to be friends (even though he gets mad when I date or talk to other guys). I told this mutual friend that I have a boyfriend and I’m pretty sure that this friend has told him that. He seems desperate to talk to me and keeps roaming around me whenever I'm alone and finds excuses to be near me. We've only talked to each other once before. I don't understand, why is he so persistently chasing me? He keeps trying to get my attention all the time even though I keep ignoring him. He doesn't want to date but at the same time won't leave me alone.
Hé has a girlfriend so why is he chasing me and being desperate for my attention?
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