I don’t know, maybe the lack of love from the other partner, one wuote I always believe in. “If you truly love your partner, give them your all, because if there’s something you won’t do to make your partner feel loved, then someone else will”.
What I feelguilty about is that I have the though of going against my morals to teach my partner this the hard way. Even though I think about it I just can’t bring myself to do it.

Neglect

Rejection leading to frustration, leading to desperation.

From what I gathered, it's that ever fleeting feeling of a new relationship. When you're married you know almost everything about your spouse and things become less exciting. Some people live for that new, exciting feeling and adore it....and are willing to risk their marriage for it....

Complacency=a feeling of quiet pleasure or security, often while unaware of some potential danger, defect, or the like; self-satisfaction or smug satisfaction with an existing situation, condition, etc.
Basically getting comfortable or in a routine

It’s a different reason for everyone. Some cheat because of sexual impulses , revenge , depression, some are just dumb yes 🤔.

Because they want to

The excitement maybe.

Monogamy. You can't cheat if you have permission.

When the other treats them like crap/ also cheats

All the wrong reasons. And I'm no exception to the rule. I tried and the person I was trying with told me to grow the fuck up. Life lessons there.

The choice not to leave their spouse but choose to still pursue outside affairs.

For some people they are just opportunists and do it because they can, some people have no morals, and some people do it because they are selfish fuckers who are never happy with what they have.

Opportunity.