My suspicion is that people who tend to want to live alone feel that way because of some experience with trauma. The baseline seems to be that people desire some measure of company, whether that be intimacy or conversation or just reassurance that they're not The Sole Survivor. When people say that they're happier without some sort of regular interaction, something has disrupted the natural inclination. Depending on circumstances, that may be optimal: if you're surrounded by abusers and people you can't rely on, self-sufficiency is clearly the approach least likely to see you hurt. However, if what I've described is an accurate model, it does mean that the prospect of an even better quality of life lingers in the background, should the happy-alone person find access to treatment for that trauma and a generally healthier and more pro-social environment.

Maybe I'm wrong though, and there are just people who would rather be alone, and that is their max healthful state. Even if this isn't the case, I think it would be wrong to assume access to those healing resources mentioned above is readily available to any given individual; it's probably not helpful to hound someone to get therapy because they say they don't like company. All of this also doesn't mean that there aren't people who feel the need to shack up BECAUSE of trauma.