"Must not all things at the last be swallowed up in death?"~Plato
Have you ever thought of Death?
Almost everything living have to go through the Reaper's scythe. Long ago, people believed that Death is caused by sin and since it is inevitable that everyone will sin, everyone will die as an act of God. Let's look at Hinduism. They believe that soul are passed from one body or being to another after death. This creates an eternal cycle where after-death goes through a process called "reincarnation". Similarly for Buddhism, Life is the purpose of entering the realm of pure nirvana or reincarnation to another being after death.
Come the 21st century, people are going away from a religious view to a more scientific view. As of right now, Death is defined as "the termination of the biological function that sustain a living organism." You die because you go through senescence, otherwise known as biological aging. There are many theories of aging, some includes chemical aging, gene regulation, etc. (You can find more about it if you browse the wiki). Since stuff becomes more scientific, this is where reverse engineering truly shines. Scientists and researchers have found the specific strand of DNA that has been causing senescence.
Does that mean when scientist finds out how to 'cure' death, all the religious theories would be obsolete? Possible. We cannot totally ignore the religious view, just as "Stars turn to dust, stardust has come to mind in human being, human being may become again stuff for stars, and untold other phenomena." We have absolutely no idea what happens to life after death, if there's even one. If, for the sake of argument, life after death do exist and human is able to achieve immortality, what will happen to the after-life then?
In the end, we still don't know shit.