Why do people date/have a thing with obviously bad choices?

Are there any answers for this in your opinion except for people just wanting sex. My boyfriend's flatmate has only been involved with one girl, who had a fiance at the time (obviously a bad egg as she is a cheater). She's broken up with her fiance and he is seeing her again. She has a low paying job which I don't see this guy being okay with as he has a good wage for his age, she was a cheater and it seems obvious to me she's just a bad choice.
Why do people date/have a thing with obviously bad choices?
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