When a person dies we cannot see beyond the corpse. We speculate on reincarnation talk in terms of eternity, but we can't see it. Death is opaque to us, a mystery. With death time ceases to have meaning. All laws of physics become irrelevant.
Death is the opposite of time.
Consequently, thinking about death brings with it fears of the complete loss of our unique identity as a person. It can be very scary to really think of giving-up everything you have become over your lifetime. You have no choice in the end. Because death involves the ultimate surrender of control, It is natural to fear death, and there are times when the fear of death can become very prominent, like during or following an illness, during a major transition in life, or during or following the death of somebody close to us. There are a number of reactions which may conceal an underlying fear of death.