"Immortality" is essentially an escape from the greed for life and the fear of death. It is understandable, but it is also a wishful thinking. It evolves into different forms due to different regions and races. For example, the existence of ghosts is believed to be because of people's fear of death. However, if you die, you will become a ghost. However, if you live in another world, you can one day crawl back and become a human again, so death will be avoided and become less scary. Ah Q's cry "Eighteen years later, he will be a good man again," is probably doing this kind of self-comfort.
Sometimes, this belief is very sad. Sometimes it's not like that.