Others earlier have mentioned evolution.

Consider how the drive to live and reproduce might have evolved from the simplest microbes. Those that responded in certain ways towards opportunities and threats would be more likely to reproduce than those that did not muster the same intensity of acquisitive or avoidant behaviours.

Since that time, survival instincts have amplified, with generation after generation being ever more desperate to compete and survive (barring colonial species with homogenous genetics). Survival of a desperate and ruthless. Now an overwhelming fear of death is almost ubiquitous amongst humans.

Our survival instinct is so great that many find it difficult to accept the idea that death might really be the end of our adventure. Ever on the lookout for an escape. Survival instincts may improve reproduction rates (noting that health prospective partners tend to be repelled by overt depressives) but they surely do not bring a more peaceful life.

It's ironic that that which increases our longevity and fecundity makes that life less worth living. Hopefully genetic and memetic evolution will sort out this problem for humanity's successors.