Why would somebody cheat in every relationship they’ve been in?

I just found out my boyfriend is cheating. When confronted he admitted he’s cheated on every girl he’s ever been with. He can’t give me a reason why. I feel like this is more of a him issue than a me issue since it been this way with everyone. But I’m confused why is he like that is there some sort of mental issue linked to infidelity?
Why would somebody cheat in every relationship they’ve been in?
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