A high percentage of them never look at what they did wrong and how they can change to become a better person.
They blame everything on their previous partner and validate everything as “I deserve to be happy”. Same with their affair partner. Even if the person was single, they could have went and dated another single person yet they were ok “dating” a person in a relationship/marriage and being a part of destroying it. Their morals aren’t too great either.
Put two of them together and what do you get? Two selfish people that are most likely used to being in a relationship with an unselfish person and expecting their now selfish partner to cater to them the way their previous partner probably did.
It doesn’t end well…