People, I find, write best about what feels familiar to them. I can't relate to your personality, but given the details provided, it might be hard to get to know to know the motivations of others.
The advice I normally give is to read authors who write characters similar to how you structure your own, while noting their development and motivations. This, coupled with an intimate understanding of your own characters (through, perhaps ostensibly trite and cliched, writing exercises provided on many writing websites) is usually adequate in creating dynamic characters with clear, tangible desires. In your situation, perhaps staying true to your intuitions is best.
Think about, for example, how narcissism causes one to view others. This could be important in how your protagonist characterizes other characters. Narcissism could be used, for example, to call into question the way the protagonist views other characters, or how other characters view the protagonist. Are others in your stories narcissists, or do they loathe them? What I'm trying to suggest here is that you might utilize the protagonist's narcissism as a framing device. It'll depend on the point-of-view of the story. Seems obvious, but it is difficult to master
An exercise, as the previous answerer noted, involves observing people and writing your thoughts/impressions/structuring a scene involving these characters and their interactions. The way you describe others might give insight into how your protagonist views others. Hopefully this gives you some direction!