All children go through a solipsistic phase in development, shortly after their consciousness is booted up enough to recognize itself as consciousness (usually around ages 2-4.) They may not be able to articulate it, but they're certain that they're the only real people in the world, and everyone else exists because the child dreamed them up. Which makes perfect sense -- the new consciousness is not yet sophisticated enough to recognize that it is both unique and exactly like every other consciousness. Developmental theory of narcissism says that something causes these children to get stuck in this phase. There's some correlation between eldest or elder of a grouping children and narcissism -- the second baby comes and takes the adults' attention, leaving the elder child feeling wounded and abandoned -- as well as abuse and parental separation. But the trauma may not be visible, or even remembered by the child or the family. (And this doesn't just apply to narcissism; all of the Cluster B disorders fit into this developmental theory, and all such disorders overlap. One of the psychoanalysts can make a formal diagnosis from a distance; I can only say this is the pattern, and this is the behavior I see consistent with the pattern.)

The insecurity of narcissism is its most salient point: the narcissist is (at least subsconsciously) convinced that he is the only person in the entire world who matters; that his opinions and thoughts are the most important; that he must believe everything he thinks, and that the rest of us exist only because he exists. However, he craves the approval of all of the non-player characters who inhabit his self-created world because he's incapable of generating self-approval and internal validation; his consciousness never evolved to that point of development. He needs their attention and approval, even as he disdains it, because what possible value can a figment's approbation have? And that is the unresolveable conflict at the heart of narcissism. Other people are both the most valueless objects in all of creation, and the most critical dispensers of positive reinforcement.

So of course he's deeply insecure and terrified of being laughed at. It's proof the non-player characters are off the script, and he's not in control. It wounds him at a deep level. People have been laughing at him - or snubbing him, or worst of all, ignoring him - for decades. He's not introspective enough nor self-aware enough to examine his own behavior and alter it. But he desperately wants approval, not ridicule, and has no idea how to earn the former or avoid the latter.

Without a time machine, it's not something that can be treated or cured. It's a fundamental flaw in the operating system of the psyche, not a result of misbehaving neurotransmitters or a broken internal dialogue.