But the catch is that in modern life, being alone more than one might like is rarely a serious survival threat. It may not feel good to consistently have nothing to do on a Saturday night, but that on its own is almost never a sign that your life is at serious risk.
Because of our hunter-gatherer past, however, being alone too much often triggers a survival-mode state in us that, like all survival-mode states, creates stress and releases stress hormones throughout our bodies and brains. And chronically high stress levels seem to be largely responsible for the physical and psychological health issues that lonely people are at higher risk for. So the cruel irony is that, although being socially isolated is rarely an actual survival threat in modern, industrialized cultures, the state of being lonely does trigger stress and survival-mode states because of our hunter-gatherer past, and so being socially isolated does often end up creating a survival risk but mainly because of chronically elevated stress levels driven by unnecessary and inappropriate survival-mode states.
The brain is, in effect, tricked typically unconsciously into unnecessary states of survival mode, such as fear of abandonment, not because of actual survival-threatening circumstances, but because our brains confuse our evolutionary past with our modern circumstances.