The Little Shaman is a spiritual coach & specialist in cluster B personality disorders, with a popular YouTube show and clients worldwide.
Now, this should be prefaced by saying that, contrary to popular opinion, not all narcissistic people cheat and not all people who cheat are narcissistic. There are people who cheat that are not narcissistic and there are those who are narcissistic who don't cheat. This is why it's so hard to say whether someone is a narcissist just based on one behavior. They are individuals just like everybody else. Narcissists who do not cheat likely look for attention from others in different ways.
For the pathologically narcissistic people who do cheat, there can be a few reasons. The biggest one is probably for attention. Whether it's sympathy, whether it's admiration, whether it's sexual attraction, attention is attention. No matter what, it's unlikely one person can give a narcissist enough attention to sustain them. They would always prefer to have more, they will usually seek more and it is more valuable coming from people who are not wise to their game, so to speak. Sex is sometimes part of the attention they are looking for, but sometimes it is an afterthought, a means to an end. It goes with the territory, you could say, but it isn't always the main goal. While this might seem like a simple reason, it really shouldn't be overlooked. Remember that they cannot regulate or create their own self-worth. They must constantly receive input from others in order to feel any. When someone wants them, they feel important. To the pathologically narcissistic person, importance equals worth. This is why they engage in triangulation. If one person wants you, you're worth something. If two people want you, you are worth more. And so on and so on.
Narcissists may also cheat for revenge. These are hostile, vengeful people who believe they are constantly being offended, insulted, dismissed and disrespected. It is not outside of the realm of possibility at all that they would engage in an affair just to hurt or get back at someone. It could be their significant other, or maybe they will carry on with the partner of a friend they are jealous of or angry at. Narcissistic people unfailingly use emotion to attack and hurt others and this is no exception.
There are some narcissistic people who cheat because they are bored. Narcissists become bored very easily and they don't experience boredom the same way that non-narcissistic do. They experience boredom as oppressive or even frightening. This is one of the reasons we often see narcissistic people engaging in constant efforts to distract themselves. They know no peace and when they are not constantly stimulated, the walls of their mind may sort of begin to close in on them. This usually results in them doing whatever they can to escape from it. This often involves bringing in other people.
Regardless of why they cheat, narcissistic people generally have one thing in common: they will not admit any wrongdoing on their part. More than likely, their partner will be blamed. This is of course, provided they even accept that it was wrong to cheat in the first place. Many pathologically narcissistic people do not. They believe that whatever they want to do is OK because the way they see it, they needed to do it. And of course, if they needed to do it, they are excused. Narcissistic people have a very hard time with understanding the difference between want and need. Like small children, want is experienced as need and therefore is very difficult for them to simply ignore. Suggesting a pathologically narcissistic person go without something they want is usually experienced by them as being denied a neccessity and they often become very angry. It does not matter to them why they shouldn't have it, only that they are being told they can't.
If your narcissistic partner has cheated on you or discarded you for another partner, please remember that it isn't your fault. People are responsible for their own actions and that includes those who don't want to admit it. All the denials in the world can't change the truth. If they choose not to learn from the things they have done, then they can go through life having the same relationship over and over again. Eventually, it wears thin but narcissistic people haven't learned other ways to get their needs met and that is often when they get in real trouble. People often worry that the narcissist will be happy with the other person and indeed, it can seem that way in the beginning. Just remember that it was like that for you too - at first. Before you knew what you were dealing with. Before you saw behind the mask. Don't envy their new victim. Even though it hurts right now, it will get better without that toxicity in your life. Pathologically narcissistic people don't change and their new victim will learn that soon enough.