I am no wiser than Yalom, so the following is taken from Yalom's "Existential Psychotherapy", 1980.
Death and taxes have been described as inevitable and although we may fear the latter in the short term, we have a deep, instinctive fear of death that only increases with age. This leads us to being risk-averse and safety conscious, taking large amounts of care to avoid harm of any kind, especially physical. This appear's in Freud's life and death drives where. perversely, we can be obsessively attracted to that which we fear most.
Often what we fear most about death is extinction, the extinguishing of our identity. We also fear our inability to control it, though we do try hard at this. Religions often offer help here, promising survival after death and control now over future heaven or hell alternatives.
We have deep needs for belonging and the esteem of others. We join groups and take on their identity, trading independence for one-ness with others. Isolation may also remind us of our own vulnerabilities and mortality. We each die alone, one at at time. With family and friends we can, in some way, live on. When we are isolated, we are also more vulnerable and any of the many choices we make might lead to hazard.
Solitary confinement is severe punishment used in prisons and at home, where being 'sent to bed' or otherwise ignored is used to bring obstinate and uncooperative children and partners into line.
Freedom and individual choice is another side of the isolation coin. When we are separated from others our sense of control is increased and we can choose and create our own meaning. We can select the safest path for ourselves, even though that may not necessarily the best for all others. We give up some freedom when we join groups to avoid isolation and death risks. Groups can also help give us meaning.
The opposite of freedom is captivity, which is what we really fear here. To be ensnared is to lose control even in the power of life and death. Freedom has been a political ideal and force for centuries. Individuals and groups have cherished it and fought for it, paradoxically paying for it with their lives as this ideal is elevated above even death.
One of the ultimate challenges of life is to find meaning in it, and we struggle to give reason for our daily actions. Meaning gives us motivation, impelling us to act in ways that give sense to our identity and choices in life.
Meaning sometimes is found on reflection as we explain what has happened. We hence spend much time pondering the past as well as projecting the meaning found there into future intent.
Religion is a very common and powerful tool for creating meaning, although it often demands some freedom in return.