Fear of Death
As humans we typically fear what we do not understand. Death is, and always has been one of the great mysteries, and it is also one of few things in life that is an absolute. No living person knows or understands what happens to us after we die. We simply know that it is going to happen... eventually. We don't know where we go, or if we will return as something/someone else. There are many religions that have ideas about what might happen after we die, but nobody knows for sure.
Some of the ideas are
Reincarnation: Where we come back as as something else. However, the what is still unknown.
Afterlife: When we stay here, on earth, in a ghostly form. Sometimes called a guardian angel.
Heaven type place: Known by many names across religions. Valhalla, the Summerlands, Elysian Fields, etc
Hell type place: Abaddon, Tartarus, Nifelheim, etc.
Depending on the religion there are also middle ground areas you could go depending on how you lived your life
Limbo: a place of neither light or dark to go and wait for judgement
Being faced with such an unknown can be terrifying to many people. I believe this is why religion tries to answer the question. With a suggestion about what may come people can form a belief, therefore removing the unknown nature of death. Which can bring peace of mind to the person that is dying as well as those around them. I have been to more funerals than I care to admit, and there is constant theme. That death is a place of peace, that the person has 'gone to a better place', or that 'they are no longer suffering the ailments of life'. These are things spoken among the living to soften the feeling of loss.
We spend so much time worrying about death and dying we stop living at all... This poison has sent many a person to the grave alone, pennyless, and crazy as can be. Now do you see the sickness/ poison in the soul here?