Just a stub for now
Aloneness and loneliness are often remarked on as being problems, and while it could be said that this stems from our ``being a social animal'' it could also be seen as a dependence on other people to help maintain your self-esteem. Dependence is not necessarily unhealthy, and if most people wo know you deliberately leave you out of their company, it is quite likely that there is some problem, either in them or in you (or, most likely in both).
It is, viewed objectively, perfectly possible to survive alone, although there is a practical advantage in that people working as a team can more easily acheive things that would be difficult for them as individuals. But apart from the practical things, what are the attributes of aloneness and togetherness, and of loneliness and companionship?
A common form of togetherness is an attempt to avoid aloneness; being alone is taken to imply being unacceptable, and so being with other people (without them trying to avoid you) is seen as a sign of being OK. People will often drop almost all of what they would stand for as individuals were they to try to stand as individuals, for the sake of being in a group. Going beyond this is companionship for the sake of feeling that you're OK, where people take on the tastes of a group so they can belong to it... look at the following of any unmusical popular music group for an example!
For a long time, when I was a chronic depressive, I saw myself as being left out of things that other people were welcome in, and saw that as a sign of my not being as good as the other people -- not being a proper person in some way -- and at the same time seeing that I was at least notionally their peer, and being very indignant about it. I could tell that part of their reason for not including me was connected with how much it meant to me to be included, and this made it all the more galling, particularly as at the time I did not understand the effect. Through pondering that situation now that I have been out of it for some time, and through pondering why I react against certain people, I can now understand that in this there is an element of using acceptance by ``obviously accepted'' people as a means to feeling acceptable, and hence the people as tools, and people don't like being used as tools. Also, the urgency, brought by this desparate hope for people to assure you that you are a proper human just like everyone else, gives an impression that the longing is sexual or at least romantic, since those kinds of longing are most commonly associated with urgency.