This was on quora by Ken Heilbrunn in response to the question:

Why won’t a narcissist leave you alone?

I am so impressed by the questions asked about NPD, and yours is one of the most important, perhaps THE most important.

I refer again to Dr. James Masterson, where Dr. Greenberg (on Quora) trained.

What you are describing is what Masterson calls “fusion.” In fact, without fusion, Masterson would not diagnose NPD. So what is fusion? I would like Dr. Greenberg’s answer to this question, but I will give it a try.

A person with NPD lives in illusions much of the time. These illusions constitute the fake image which the NPD creates by himself and presents to the world around him. They are what is referred to as the “false self.”

The illusions are partially real, since they draw on real things in the NPD’s life. But they are embellished and exaggerated to the point of being more false than true. Like all illusions, they require constant/repeated maintenance. Otherwise they pop like soap bubbles and leave the NPD exposed to the truth of what he is. And that is something he cannot deal with. You can deal with who you are, genuinely are. He cannot. Ask him to and he will explode.

In fact, without these illusions, he feels — actually FEELS — that he does not exist. Think about that for a moment. You KNOW that you exist. He does not. You do not even question whether or not you exist. Not so for an NPD. He feels that he exists only when he is under the spell of the feedback he craves from you and others. And even that FEELING is an illusion. What’s more, deep down inside, he knows it. Think about it again. What an awful state to be in.

It’s a life sentence of solitude without parole. Solitude despite his fan club. Like the Greek god Sysifus, who was punished for his self-aggrandizement and craftiness by having to role a boulder up a mountain, only to have it crash into him before he got to the top, forcing him to start over again, for eternity.

That’s why he can’t leave you alone. He is an emotional parasite who is dependent — or thinks he is dependent — on a symbiotic relationship with you. You are the life blood of his mental state. Even if he triggers negative reactions from you, at least those reactions feed his need to feel that he exists. It is a bottomless pit. Fill it a bit today. Tomorrow it will be empty. He will come back for more. Like a drug addict. Just remember: He can’t help it. He can’t shake the habit. Not by himself, he can’t. And you can’t help him, much as you may like.

That, as I understand it, is fusion. It goes to the heart of the disease. Without that parasitic component, Masterson would not diagnose NPD.

I wish Dr. Greenberg would comment. I value her expertise.