Affairs cause a great deal of shame for both partners. The injured one thinks that their partner betrayed them because they are not good enough. Maybe they are not attractive, charming or special. They conclude that there is something missing about them that their partner found in the other person.
The partner who had the affair also feels a great deal of shame. They see the pain they have caused their partner and they feel horrible about that. Since they were the ones who cheated, they see themselves as immoral and dishonest. Their partner's attack and criticism intensify their shame and cause them to react and become defensive. However, when they react, their partner thinks that they are not taking ownership of what they did and consequently attacks them even more. It becomes a vicious cycle that prevents partners from healing.