I've found myself in Wikipedia. Ironically.
A loner is a person who avoids or does not actively seek human interaction.
Some people see it as beneficial- maturing much faster, seeking knowledge, reaching their goals easier, and more focused on the task at hand. Disconnected from the people around them, they are more likely to make their own decisions and go miles ahead of those around them. They relate only to individuals they consider worthy of their time. Therefore, this type of loner will have very few intimate relationships.
Loners may socialize greatly with those in whom they can confide. If someone unknown to a loner enters the social group, the loner may automatically shell up. Insecure loners find it excruciating to be in the physical presence of others. Anxiety is a common feature of their social interactions.
Most loners are able to act "normally" in a social setting. However, the strain of being in a situation which is uncomfortable may leave some mentally and emotionally exhausted. They may have to retreat for a significant amount of time before being able to do so again.
Source: Loner, http://en.wikipedia.org/wiki/Loner
