Why does a guy play childish ‘make her jealous’ games when he chooses another girl over you?

I mean he liked and flirted with us both. He thought I rejected him many months previously as I didn’t realise him asking me out was a date per se. He started dating this other girl and even still sent mixed signals for weeks and months after it. He says things and you’ll have to take my word for it but explicitly to make me jealous. He stares, teases, eavesdrops when I talk to others. I’m not anyones second choice. I’m not anyones back up. We don’t really text these days other than the rare communication in groupchats. Why does he still try to push my buttons or get under my skin. I’m doing everything I can to avoid him but we’re in the same circles and I will bump into him from time to time. I broadly ignore him but even then he tries to get my attention. He could be all over his girlfriend and he’ll be repeating things I’m saying in my conversation with others. He’s wrecking my head..
Why does a guy play childish ‘make her jealous’ games when he chooses another girl over you?
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