Can someone help me understand why people cheat?

I have virtually no social skills and I suspect that my social IQ is next to nothing. I also have absolutely no experience in relationships.

I just don’t understand. Infidelity in all forms have always eluded me.

What is a person’s motivation for going behind their partner’s back to sleep with other people? Also, what is a person’s motivation to engage in polyamory or to have an open relationship?

I don’t want to offend anyone I’m just genuinely trying to understand this.