I just got out of a relationship with a narcissist so from my experience, another reason would be that if that person is a narc then the ex’es give them a supply source for ego stroking and a way to make the current partner jealous and more submissive (triangulation). You would see other narc symptoms if that is the case, such as blame shifting, gas lighting, self-absorbed, never thinking they are wrong, etc… anyway, just another possibility..