Some loners have become a little too self important and take life too serious. They will do anything to avoid anything which may upset them as they think they cant handle emotional pain. Being in groups of people is too risky as well is having a partner, usually bad experiences from the past cause them to make sure they dont risk their emotions again and try to cocoon themselves with a false sense of security. Their social skills sometimes are poor due to lack of experience and this nervousness can create barriers as it tells other people that there is something wrong with them.

They can change this though through various techniques, learning not taking life or themselves too seriously and developing a confidence to handle the emotions they fear being at risk from.

Obviously before anyone takes this personally, this is just an observation from experience with people who class themselves as loners who don't want to be a loner any more. Not all loners are as described above, I am just mentioning one aspect of something I see on a regular basis.