In relationships generally, there are predators. These are people who go after someone vulnerable, and exploit them. They want something and they'll scheme and manipulate to get it and leave a trail of destruction in their wake. The same can apply to affair partners.
My experience, and keep in mind I'm an old bird who says it as I see it, is that men quite often are after a shag. They'll take it if they think they can and a lot of the time their dick does the thinking. They'd not be above flattering, pursuing and going after a married woman.
Women on the other hand are often looking for more than a shag. Quite often it's some kind of relationship and I know for a fact some will go deliberately after married men if they exude money, power, confidence or even represent a chance for her to prove her worth by stealing someone else's man.
This is not how most people are, but in my years of life I've come across these things. So if your partner has an affair the AP might be a predator, same as any other relationship. They also might just be a shit person or someone who's desperate themselves that pursues someone married.
The point is really more about your spouse. I think many people are capable of having affairs for a lot of reasons. Sometimes they're lonely or susceptible to ego boosts, sometimes they're just horny and selfish.
Each affair, like each relationship, is different. What's always constant is that it requires a great deal of selfishness to do it, and it's always, always, always the spouses choice to say no.