As I spend more time getting to know people, I think I'm starting to understand traits we all seem to have in common. To some degree or another everyone is in constant pursuit of acceptance by their peers. Take dating, for example. The largest compliment one can receive is that someone of the opposite sex likes you. Apart from a superficial crush, someone who really knows all of you and likes you for who you are really builds your self-esteem. It's total acceptance of you. Yet we often spend months getting to the point where we trust the other person enough to show him/her our true selves. That way there's less risk of being (and being hurt) by rejection.
Even in non-romantic social spheres, people crave it. I used to be highly turned away from my synagogue's youth group because I hated the pressure to be the cool kid, and everything you had to have and pretend in order to be so. But outside of the popular spheres, we all share this basic need. If we can define a group in which we belong, we feel automatic acceptance. When groups clearly define themselves, they end up narrowing their circle of acceptance. Goth kids in school still get derided for not being alternative enough, liberals at a liberal university get mocked for not being progressive enough, Christians get snubbed for not acting Christian enough, etc. Even in non-official groups, people change their behavior when they're in a group. Now, some are more subtle than others, but if you know someone well enough, you can see the front they put up to increase their likelihood of social acceptance.
Perhaps this is why good friends are so important to have. They're defined by their ability to see you in your entirety (or near entirety), really know you as a person, and still enjoy your company. Now, this doesn't mean each person needs to have a hundred-person network, yet they do need at least 1-2 people deep down they really trust. It's why while I think the terms extroverted and introverted can really describe a person's personality, the term "loner" is a misnomer. Those who define themselves as such are really just trying to convince themselves and everyone around them that they are above this basic human need. That way they won't be disappointed if someone fails them. In reality, this self-protectionary measure is a social mask they wear full time. Perhaps, these people are the ones who need a friend the most.