This is an interesting topic. Why are they nice? I find myself thinking about HOW they are nice. In my experience, they tend to perform acts of service. They DO things. I think that they do this in order to try to keep things on an even keel, and feel good about themselves. I realize that each person is different, and even each N is different.
Some people are not N'ish unless they are backed into a particular corner that triggers them--and then they go for the throat. The fact that they only do it occasionally doesn't make them less dangerous.
My H is diagnosed with N tendencies, not full-blown NPD. He does all sorts of things for me. Brings me OJ and coffee often in the mornings. Makes meals lots of the time. Cleans up. Does lots of things around the house and yard. So his actions are very kind, much of the time.
But his words... Horrible. He will actually DO all these things for me, or us (the house and yard items), and then COMPLAIN about it and try to make me feel bad. I have told him this makes no sense. I say that if he does all these things, and then complains endlessly, it's as if he did nothing. The pleasure of having his help is negated by the unpleasantness of his griping. It's a wash. Just as if he had done nothing. I don't think that he is able to stop himself from complaining.
You all are really helping me. I don't mix it up with him anymore. In an argument, he becomes irrational to the point where I have ask him if he's being serious or joking. I have lived most of my life around lawyers, and arguments are not a reason to get all worked up, IMO. You just present your best case, and in the end somebody stands corrected, or else you have to agree to disagree. But if you get all furious and stupid, make absurd generalizations, etc., you forfeit. Loser. Say that to my H, and you can make him super-angry. It's like pushing a button and watching him dance. But I have stopped doing that. I have stopped arguing with him. Thank you.
It's very weird because, as noted elsewhere, they really want respect. But their tactics are so bizare that over a period of time they lose the respect that they had. They inadvertently work against their own objectives. I think they're nice to try to do something to gain back some ground, having lost so much in ways that they really can't figure out. It must be so strange, living in a total of such total denial. I watched our T listen to H talk the other day. The T's mouth was hanging open in astonishment, and he's one of those super-T's that are hard to get in to see. He has that kind of background where they understand really odd motivations, like the pros on TV crime dramas that can tell you all about the criminal who would do a certain crime. H was astounding our super-T. But H would say it didn't happen that way. He'd say that's just the way I'm spinning it. Funny about denial. It is the ultimate defense, because nothing else matters.
I do feel sorry for H. Anyway, I'm trying to just appreciate the things he does that are nice, not argue with him, treat him as well as I can, and work on my own personal growth.