One of the questions I get asked is, what is the motivation of the other person? The other man or woman to engage in an affair? Particularly, if it's an affair with a married person. And there are numerous, numerous reasons why a person would engage in an affair with a person who is in a committed relationship. Everything ranging from an opportunity, to material interest, to a fantasy that that relationship will dissolve, to simply boredom and sensation seeking on their behalf as well.
The aggrieved partner's spouse usually vilifies the other man or woman; however, from a human perspective that other person is simply a fallible human being. They might have believed the fantasy that the cheating spouse presented or they might have deluded themselves that it was a relationship worth pursuing. What you have to remember is that the cheating spouse or partner presents a very different story of the relationship that you're in than the one you would present. And that is, the other man or woman hears a completely fantasized version of that relationship, perhaps the one in which the cheating person is being misunderstood or abused, and hopes that this affair will lead to dissolution of that relationship and to, you know, maintaining the new relationship.