Some people have a personality
unsuited to making friends
. Here is the story of the "Hedgehog's dilemma"
If a hedgehog wants to give his warmth to other hedgehogs he must get near them, however the closer he approaches the more they injure each other. It's the same with some people. They feel that they can not form relationships with others, and can't let people close to them, for fear of being hurt.
Some people let this fear rule their lives, they won't enter into any relationship because they fear what they perceive as the inevitable break down of that relationship and thus feel that they would be better off not having it at all.
We live and grow by interacting with others, and it is other people that hurt us the most, but without others then we are the same as nothing (see the last 2 episodes of Neon Genesis Evangelion).
People cause us pain, by avoiding people we avoid pain, for a time. However loneliness is a type of pain also, and it can grow to become more than the pain caused by other people, therein lies the Hedgehog's Dilemma.