Isolation often gets a person stuck into positive feedback loops of negative feelings (loneliness, anxiety, sadness, guilt, shame, cynicism, paranoia, listlessness, self loathing, despair, etc.), disrupted circadian rhythms from poor sleep and lack of sunshine, lack of routines and loss of time sense, lack of exercise, unhealthy diet, poor personal hygiene and cleanliness, substance abuse, financial problems, and so on.

Without sources of external correction/feedback/perspective, even tiny negative thoughts can get amplified and spiral out of control, and a human can stop behaving within the range of personhood (that is, the usual social construction of normal behavior). Without any human contact, it is difficult to keep control over the focus of attention, find meaning in daily routines and projects, etc. Even trivial tasks can start to take monumental amounts of mental effort. With sufficient sensory deprivation someone can become dissociated or start hallucinating, or can fall into catatonic depression.

Some people are obviously better at handling isolation than others, but it’s tough for even the strongest willed. Involuntary isolation (e.g. solitary confinement, or long-term homelessness) is a torturous experience for most people, often causing permanent emotional scars. Voluntary isolation (living as a hermit in the woods to write a novel or whatever) should not be undertaken lightly.

Often people isolate themselves after some kind of severe negative shock, e.g. the death of a loved one, a bad break-up, a professional failure, etc. People experiencing strong negative emotions without any social support/comfort from family/friends can get themselves into situations that are difficult or impossible to climb back out of, especially if substance abuse is involved.