When the experience is over, when this set of desires gets exhausted by experiences the body is dropped. So, death is a natural course of events in the process of evolution. Life and death mean one and the same thing, in fact, and the question of Nachiketas, and the answer of Yama in connection with this question, are not aspects standing apart, but form the obverse and the reverse of the same coin. While the first boon pertains to a possibility of the return of the soul to this very same world, the second one relates to rebirth in a higher world. Rebirth is unavoidable, but it need not mean that one will be reborn only in this physical planet.
Experiences are endless.
The Universe is not exhausted by the earthly experience alone. Fortunately for us, all that is corroborated by the modern physical theory of relativity, and the mathematics of modern times has merged itself in the philosophy of the Upanishads. That is wonderful. When we reach the apex of knowledge, we come to the same point. The relativity of experience is an explanation of the inner connection between life and death, but the ultimate meaning of death as well as of life, which is the meaning of the entire evolutionary process, is the Self-realisation of the cosmos.