Why do boys cheat in a longterm relationship?

I recently found out that one of my male friend is cheating his girlfriend. He is in a relationship from past 9 years, his girlfriend is very sweet and simple. She loves him very much and unconditionally and she also trusts him but he is cheating on her physically. I don't know reason , why does guys cheat on their partners who are serious for them and love them unconditionally.

Why do boys cheat in a longterm relationship?
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