Narcissists only believe in themselves, by Last's definition. Other people are supporting cast and villains in a movie the narcissist stars in. The chick in the movie might leave the hero, either because she was fooled by the villain or is really a villain all along. Unless you see people as having their own identities, you can't know that person well enough to trust them. (Well, I suppose you could use a different world-model than multiple identities, as long as it's more accurate than a personal narrative, but Last doesn't discuss that.)