People cheat because they consented to impossible promises. Because one person can not be all things to someone who wants a person to be all things. Unreasonable expectations. Breaking a contract is not uncommon as we have come to learn from a certain president. They cheat because hormones are racing & the moment carries one or two or 3 , away. Because the feeling one gets is irresistible as chocolate. Because They don't care. Because mind altering liquids were ingested. Because It is a question of Mind over matter. I don't mind & you don't matter. Because you suck Because I suck Because we won't suck. Who the fuck knows. Pick one.

That's a good question. There's probably a lot of reasons, but a common thread among all of them is that they don't fundamentally know and/or care that it's wrong.