why do people cheat on each other if they like their bf/gf?

it doesnt make sense to me like i will never understand why a girl will cheat on her bf but will cry the next day because he broke up with her like wtf did she think was going to happen he was gonna stay with her and she never even went out with the guy she cheated on her bf with like wtf its just so stupid someone explain this to me because this happens to alot of girls i know?