Does cheating primarily happen in damaged relationships that give off certain symptoms? Or does it happen out of nowhere to unsuspecting good people that their SO cheats on them?
cheating is never out of nowhere, it's to be expected. the question is how you deal with it
Both
I'd even say that it mainly happens when each partner has one of the perceptions you mentioned. For instance, the guy or girl is unhappy with something but refrains from speaking up for any reasons, and proceed to cheating. While the other, unsuspecting part, (rightfully) thinks he/she was doing everything right