"don't most people just feel fine the way that they are and comfortable around people by default? "
I don't know about most people. I know that for much of my life I didn't feel fine they way I was and I was not comfortable around people. Today I still get a bit uncomfortable when around complete strangers. I suspect that had/has a lot to do with upbrining. I think it's something that can change and I think that changeability is a default human feature.