About mortality
One of the main topics of the mythological beliefs of any ethnos is the topic of human mortality. People imagined gods to be the immortal creatures unlike themselves and tried to solve the mystery of their imperfect body that was dieing day by day. The human being is the only live creature that realises he/she is going to die one day. Other creatures don't even imagine they really may die. That is why it is naturally that people started to pay particular attention to human mortality, as they lived knowing they are approaching to their end.
The natural desire to live was the base of people's views on the death and life after death. People thought death is anomaly that somehow steals the human lives so that even gods can't protect people.
The explanations of this phenomenon vary in mythological beliefs of different countries. And these explanations mixed as all countries of the world were interconnected and people exchanged their views from country to country.
The mortality has not been explained till nowadays. People of the world know they are going to die one day and that everything they do doesn't have any sense. But people got used to live with such understanding. And this topic of human mortality was reflected not only in the ideas of people on the other world, but also in the beliefs of people in different magical creatures and spirits who lived there and the gods who ruled it. And in the result all these traditions were united into the religion that combined all ideas of people about the other world.
So mortality was the base of the human ideas on the other world.