This is an internal assumption based on life experience and basic deduction from numerous theories that I’ve read. There are some theories that suggest no one actually dies and the life you live will inevitably lead you to parallel universes where you survive for hundreds of years through technological advancements all while you see others “die” but actually they go onto their own parallel universes. You can also deduce this from the coming singularity that you won’t be able to die you will just be eternally tortured.
