If he thinks so highly of her, then why cheat on her?

why would a guy whos successful who talks highly of his girlfriend saying how great her personality is, how she has great work ethics, that their personalities go great together they both like the same things. He made his girlfriend his business partner, Then he turns around and sleeps with some other girl that he's been friends with for awhile. She's also his employee and this girl was married to someone else at the time. I need everyone to focus on why cheat on someone you claim to really like and speak highly of. Whats going on? No it's not about me.
If he thinks so highly of her, then why cheat on her?
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