Why would someone be this immature?

So, I just learnt that a guy I thought I was friends with, deliberately lead me on and let me fall in love with him to pay me back for a small disagreement I had with one of his female friends, even many more years ago. One of his friends told me. Years later. I’m not at all upset. I just don’t get why a grown man is his thirties would behave like that. I think it’s silly.

Why would someone be this immature?
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