Narcissists are known to cheat on their partners. They are self-absorbed and lack empathy. After some time, you lose your potency as a source of supply. They get bored and they are no longer interested in you. This has almost nothing to do with you and everything to do with them. They can never be satisfied, so they need to look for a secondary source of supply. The secondary source of supply is often used for sexual purposes, while as the primary source of supply, you are used as their emotional punching bag. Each source of supply only has a certain amount of life or potency, until it can no longer satisfy them. The narcissist always gets bored of whoever they are with, especially when they start confronting them or blaming them for something. They don't want to be responsible for anything. When you start confronting or questioning them, that is when you go from being the ideal source of supply to being worthless or
Narcissists have a dualistic mentality. A person is either all good or all bad to them. By confronting or questioning them, you have now gone from being all good to being all bad. Which means they now need to put someone else in the place that you were, as the idealised source of supply. Narcissists do not honour loyalty. They do not honour love or enthusiasm for anyone. All of their enthusiasm is for themselves. They believe that you are meant to serve them. Your needs do not matter to the narcissist, so they will cheat on you. But they expect you to remain loyal to them.
Narcissists are desperate for attention. They are always seeking narcissistic supply and they will do whatever it takes to get it. They will put themselves in positions where it could be humiliating or degrading for them, out of desperation for attention. Out of desperation for feeling as though they are worth something or as though they are desired by someone. They are very insecure. This is why you should never invest anything of significance in them. They will always fail you in the end.
When two people have a relationship, it is an agreement where you can invest your time, energy and money in building something with this person, without the fear of having it compromised by something else outside of it. When a person cheats, that is a breach of the contract and the agreement which was made when they got together. This is why people choose to have relationships. But narcissists do not honour this contract or agreement. They change the conditions of the contract or agreement, without considering the other person.
The narcissist isn't interest in investing their time, energy and money into anyone, unless it can benefit them in some way. They will do this in the beginning, to secure you as a source of supply. But once they've got you, they stop doing the things that first secured you. They are easily distracted. Once they have got what they desired and they know it's not going anywhere, they no longer desire it any more. They don't have the qualities, skills or attention span to make something grow to a significant or meaningful level. Once they know that they've got you and you're not going anywhere, they're looking for other sources of supply.
In their minds the relationship isn't real, it's just a part of their fantasy. They play their role, they wear the mask of their false self. And in their minds, you have a role too. When they're looking for another source of supply, that new source is a part of the narcissist's fantasy. This is in their minds, none of it is real, so they can change it whenever they like. They don't consider the consequences that this may have in real life, or how it may affect the real people that they are dealing with. In their minds it's just a game and they're not dealing with real people. They're just playing with objects, like a child playing with their toys.
Narcissists are very unpredictable. When you are around them you will always be in a state of confusion or uncertainty. They have no morals or values and they are always seeking attention. They need excessive admiration, so they can never be loyal. They are not suitable for a significant or meaningful relationship. They are always seeking narcissistic supply. They are always seeking attention and admiration. So inevitably they are pathological cheaters.
If you are with a narcissist, don't expect them to be loyal, honesty or trustworthy. Don't expect any dedication or devotion. They will cheat on you. If you are with a narcissist, they have probably cheated on you already. Whether it is emotional or physical cheating, they are cheating. They are incapable of being loyal, which makes it impossible for them to be committed in a relationship. They don't understand the value or significance of what you have to offer. They are always looking for something more or something better and to them, the grass always looks greener on the other side.