I have been intrigued about this topic, Ygrec, since you brought it up, and I have been pondering it. However, that said, I am still sort of talking off the top of my head in my response and might even ramble a bit. But here are some thoughts I have:

I think humans were created to need each other. In the Biblical account of Adam in Genesis, he expressed that he was lonely, since no other creature God had made met his needs. So, God created Eve.

Whether someone takes this account literally or as a myth, I think it is correct. Man is not meant to be alone.

That said, I do agree that early childhood experiences play a major role in how we end up relating to other people. For example, children who are abused might end up with a lot of mistrust of others and feel more comfortable being alone. Also, though, children who are abandoned, literally and/or emotionally, might develop borderline personality disorder. I know from experience that people with BPD often do not want to be alone and that their major fear is abandonment. And, the lifestyle that we get comfortable with as a child can certainly carry over into adulthood, as in the examples given of children who spent lots of time alone and whose parents did not encourage emotional intimacy.

My belief is that the most emotionally healthy people can be comfortable being alone, because they are comfortable with themselves, but they also enjoy sharing themselves with others. Maslow said that "self-actualized" people did have only a few friends, but they were close to them.

We all know the stereotype about "loners." Most every time we hear about someone who does something like kill a bunch of other people, that's the way they are labeled. I'm not sure if they started out just being loners or if their unusual behavior turned other people away.

I can't say how I would react if I were in solitary confinement for a long time. In fact, I don't know that any of us would know for sure until that happened to us. I do know that prisoners of war who are separated from others often report that they survived by thinking about the people in their lives.

I do believe that our individual (innate) personalities also play a role. As was mentioned, for example, the traits of introversion and extroversion could be factors. Introverts need some time alone to recharge their batteries, and the opposite is true of extroverts. But, still, I think both types need time with other people. BTW, introversion is not the same as shyness.

As Barbara Streisand sang, "People who need people are the luckiest people in the world." I believe that the ideal is to be able to find some "kindred souls," who understand us and accept us. Friendship is generally built on mutual interests. I do find that being on PC helps me because I can relate to folks who know what it's like to be mentally ill and accept that aspect of me. I can be myself here. I have close friends IRL, though, some who are "normal" and one who is also bipolar. I can talk about being bipolar more, though, with my friend who shares that disorder.

Well, these are my random thoughts. Maybe I won't be ostracized for my remarks!