How much is loneliness a result of people isolating themselves as a result of fear of rejection? A more tolerable form of suffering than being overtly rejected when one reaches out to others for desired human contact and affection?

This indeed is a major source of difficulty that leads to many failures in life as well as simple unhappiness being a constant feature when people have inadequate social contacts because they are in a vicious cycle of fear and avoidance to limit the damage by anticipating rejection and then cutting themselves off from the possibilities of making friends and being fully accepted—as in a school or work setting. So this is a major liability and risk factor for individuals who develop this pattern of emotional responses as it can trap them for a lifetime in self-limiting beliefs and behaviors as a consequence and they may never break free. Again, you are looking at the problem of social isolation in isolation ironically. Where are loving humans who will fully know when someone is feeling awkward and hanging back but do nothing to help engage the person to reassure them and offer initial acceptance as an act of friendship to help draw them out and draw them in so they can be a part of the group? This is the humane and human thing to do. When it does not happen, it is because people have become conditioned to avoid anyone with a liability that is discernible fearing that it may rub off on them by association. If they are friends with a weak link in the group, they see it as a lowering in their own status. This is the poison introduced by having a pecking order where some are favored and seen as high status, high valued associates and friends, because they have power in the hierarchy by virtue of authority bestowed on them or personal attributes that make them appealing such as good looks, being prosperous financially, having the right kind of personality that makes them sought after as friends by others or have a leadership role in a clique of followers who gain from being members of a select group, but this isolates them through the need to separate themselves from the non-members to maintain their presumed superior status in the hierarchy. These divisions take a toll and are a major liability in society when it comes to fairness and the availability of love and explains why many are left out.