Silent Treatments, Smear Campaigns, and Rejection

Getting ostracized from the group is a real fear for a primate that is based on actual survival. Humans are Great Apes (Hominidae), just like the others; Chimpanzees and Bonobos, Gorillas and Orangutans, and we all live in social groups. These groups are our protection, we work together to keep alert for all manner of dangers, and we work together for resources like food and water. We protect one another from predators, and from violence within the group. Humans evolved in the natural world surrounded by the same dangers as the other apes, requiring the same resources and protection. Over time we figured out fire and weapons, and that gave us some freedom as individuals, we could survive away from the group for some time, but it's much more difficult and much more dangerous, most of us have remained grouped with other humans, relying on the relative protection and resources that a group provides. We also rely on the social and intellectual interactions within the group for our mental and emotional well-being, which is a very large part of who and what Great Apes are, how we're built.
When a member of one of these social groups becomes unacceptable to the others because of severe and erratic behavior, he or she is shunned, driven out and away by either the leader or the whole group. When a member gets separated from the group, he or she is very vulnerable to predators, the elements, and other dangers. Especially in youth, all Great Apes are very vulnerable, and seem to instinctively cling to the group, they don't stray far. Individuals from one tribe or clan are not necessarily welcomed in another, and may be driven out into dangerous territory, sometimes killed, or simply unaccepted, and therefore unprotected, ignored, and not allowed to share resources.
Being shunned, being unaccepted, losing approval of others in the group can mean serious survival consequences for any group animal, but especially animals without horns, claws, and large teeth. We can't fly and we can't swim fast, nor can we run fast compared to most predators. We don't have thick fur, either. Great Apes can "run" and/or climb, but that's about it; only humans make weapons like spears, bows or knives, but if we don't have one in our hand at the time, that point is moot. We need the acceptance of our group in order to survive on Planet Earth, especially if we have young ones to care for, and when we are young ones ourselves, or sick, injured, or weakened.
So when another member displays shunning behavior toward us, such as cold shoulders, silent treatment, ignoring, invalidating, rejection, humiliation, and smearing (getting the group to reject us), our subconscious knows that we are in actual danger. And we are also at risk for losing our intellectual and emotional stimulation that we need for our mental health and well being.
Even if we choose to live alone on the side of a mountain, there is a huge difference between being able to go back to human groups when we want to, and being ostracized by those human groups. A human who has been shunned does not receive resources or assistance, is often kept from getting his or her own resources, and is much more likely to be mistreated by members of the group.
So when we have a serious emotional reaction to people who display shunning behavior, we're not crazy, we're human.