Yeah, we’re pretty judgemental people. Introverts are just socially immature people who haven’t yet learned to integrate. Stuff like that.
Let’s take this via reductio ad absurdum. Would you say a man with Down’s syndrome is just someone who hasn’t chosen to behave like other people? Just hasn’t applied himself? Just hasn’t learned to deal effectively with trisomy-21?
Well, “that’s genetic,” you say. “Nothing you can do about that.”
Ok, that’s valid. But it’s only genetic because we’ve seen the genes and can tell that something is different there. Are you absolutely certain that introvertive tendencies aren’t at all genetic? How about hair or eye color? Hard to control those, isn’t it?
If someone looks normal on the outside, we have an expectation that they will behave normal, feel normal, and think normal on the inside. And we believe “normal” means “like us” or “like most other people I’ve met”.
Here’s something to consider: many autists, who appear different and behave different on the outside are normal on the inside. The autism behavior, for them, is a kind of language they’re using to interact with the world.
“These children were just of another kind,” [Laurent Mottron] says. “You couldn’t turn someone autistic or make someone not autistic. It was hardwired.” In 1986, Mottron began working with an autistic man who would later become known in the scientific literature as “E.C.” A draftsman who specialized in mechanical drawings, E.C. had incredible savant skills in 3-D drawing. He could rotate objects in his mind and make technical drawings without the need for a single revision. After two years of working with E.C., Mottron made his second breakthrough—not about autistics this time but about the rest of us: People with standard-issue brains— so-called neurotypicals—don’t have the perceptual abilities to do what E.C. could do. “It’s just inconsistent with how our brains work,” Mottron says.
From that day forward, he decided to challenge the disease model underlying most autism research. “I wanted to go as far as I could to show that their perception—their brains—are totally different.” Not damaged. Not dysfunctional. Just different. 
We don’t take savants with extra-normal—even desirable—abilities and say that the rest of the population is retarded. We recognize that savants are different. We don’t take very ugly or very beautiful people and say they just haven’t learned how to look normal; we know that they’re just different from the rest of us normal looking people.
So maybe we should stop looking for differences in others, labeling what’s normal and what’s not, and just start understanding that “normal” means nothing at all when you’re talking about a population where no two people are alike. Especially on the inside.