Death is a constant in the universe and whatever lies beyond it. It is an inevitability that almost every living organism faces, though some are seemingly completely immortal or completely invincible. What causes death in an organism can vary from ageing, disease, or being killed from an accident or an attacker. Whatever the reason, death comes to them all the same.
Other examples of things that are seemingly exempt from death are Shadow Beings, the things that challenge the very concept of a living beings(s)
Some intelligent species and civilizations, like Humans, Zythyn, the Etymology, and so on, discovered ways to make themselves virtually immortal by using various methods to stop the ageing process. And with advanced medicines, most diseases will not effect these aforementioned species. However this does not stop death in its entirety, as they are still susceptible to dying from accidents or being killed.
Other ways intelligent species avoid death is to upload their consciousness in various machines, allowing them to move themselves across millions of light-years in mere seconds to other machines in the universe that they can connect to. Though this only seems to prolong death rather then stop it completely, as they can still be killed if all machines that they can travel to are destroyed.
Death is not exclusive to living organisms either. Planets, moons, stars, black holes, galaxies, galactic super clusters, and so on, all seem to have a life span as their energy will eventually dissipate over impossibly large amounts of time, which can be described as a form of death. In fact, it seems that every piece of energy in the universe will eventually fade.
To answer this Eternal Question, many intelligent species create religions and belief structures. These usually have a common idea, in having an all powerful creator(s) that orchestrate the infinite universe in a specific way unique to that creator(s). And when they die, then they usually will create an afterlife that this creator(s) sends them to after their death.
These afterlives can vary depending on those who invent them, however a common trope in these is that their are various afterlives based around the actions an individual does during their lifetimes. For example: One may be an infinite reward, wherein the individual lives in an infinite state of bliss and completeness. While another might be an infinite punishment, wherein the individual lives in an infinite state of agony and sorrow. What makes an individual enter within these specific afterlives varies depending on the religion in question.
These are usually created to serve as a kind of comfort to those who fear their own mortality, which seems to be quite common in intelligent species. The exact reason an individual is afraid of the concept of death can vary of course, but they usually seem to be afraid due to the unknown factor of death. Knowledge is a constant need in almost every intelligent species in the universe, so not having, and never having that knowledge scares them.
Why death is a constant of the universe is a question that cannot be answered it seems. If the universe came into existence from nothing becoming something, then there probably isn't a reason it operates the way it does. However, since the life and death process is as old as time, then there is most likely a reason it works the way it does. And If the universe was really designed by a being or beings, then how or why they make it operate the way it does I suppose would be only known to themselves.
Despite these questions, death seems to be one of the few practical certainties in an uncertain infinite universe. But, since the universe is infinite as I've previously mentioned, and that the multiverse is proven to exist by most species at this point, then maybe...perhaps...all intelligent species will one day know how and why death operates.