What drives people to be unfaithful, to be so disloyal to someone they claim to love that they will share themselves with someone outside of their relationship, commitment, marriage.
Selfishness?
Lack of self-control?
Fear?

I would like to venture that it’s not the act in and of itself that is the most painful to that person’s mate. It is the deception, the lie, the failure to be honest about where one was, what they were doing, the distortion of the who , what, when, where, and why. And that deception is not the actual act, but the source of the most difficult result of the act. And what drives that…
Ego.

The sheer audacity to assume that your truth will remain hidden, when we ALL know that the truth always comes out… in some form or another… always. That’s the work of the most irrational, asinine, and self-protecting part of the psyche.
It is not the fault of being ignored or feeling lonely. It is the fault of audacity.

The complete destruction of trust and safety. The realization that this person you share a life with, space with, who in your most vulnerable state you sleep next to, would sacrifice your shared trust and your personal safety in the name of protecting themselves, over your unit. The audacity, the deception, the egotism… that is the undoing.

Be responsible. Honor your commitments. Be strong enough to communicate through whatever fear, lack, challenge, temptation might be at play. Don’t sacrifice your 80 for a dalliance with 20, and there will be nothing to undo.