Why do people cheat? or choose to be side pieces?

It makes no sense how you can cheat on someone. Or why you would knowingly be a side piece. It really hurts this happened to me. My now ex was cheating on me with a girl who knew he had a me as a girlfriend. She was so unable to see outside herself and her wrongs. When i found out she said “this is annoying how she’s always bugging about stuff. And if she really has a problem with me, send her my way”. Like what is wrong with people?
Why do people cheat? or choose to be side pieces?
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