Only means you haven't met others interesting enough to share thoughts with .

I think it's simply an individual difference, like some people are good at math, others at dancing, etc. I don't think being an introvert is a bad thing at all. If you regret the social opportunities you missed out on as a child, try to find substitutes now. Go to a senior citizen center in your area, if there is one. Take non-credit adult education classes like learning to cook Greek food or travel or whatever. But don't think you are flawed or damaged because you are an introvert.

I don't know that there's any particular cause. It's just the way you're wired. My brother wouldn't miss a party if his life depended on it, but I agonize every time I'm invited to one.

I was born this way. Can't speak for anyone else.

Being an introvert isn't something that is forced upon you. Being picked last at games (me too) didn't cause it, although it didn't help either. You weren't the outgoing member of the "in" group or the kid that not everybody knew everything about so you were left on the outside looking in. You were you and not like them and they couldn't understand that. The army (thanks for your service) helped because of the structure-everybody's in the same boat. But for the most part it's just the way your wired, and if the rest of the world doesn't like it-screw 'em.

I don't think anything makes you an introvert. I would go along with those who think you are born that way. I was an introvert from when my earliest memories were formed.

You make it sound like introversion is some sort of a disease or character defect. It is just personality, one part on a continuum.

For me, was the death of my mother when I was 7, being left in the care of my aunt and uncle, with a father and 3 siblings (at the time adults) still living. Started me down the oath of lack of self worth that turned into, among other things, and introvert. I've taken many strides to over come that as an adult, but I still revert to introvert behaviors.

I am sorry for your trauma. I am happy tho that you have healed the wounds. When I came to accept that society was mostly dominated by hypocrites & miscreants, I bailed. My animals never did the things to me that humans did to them as rescues. I can not abide most people. Such is why I am an atheist introvert rabid environmentalist.