That's a question people have been asking themselves for a while, and no one really can answer. All I can say is that, though people more educated than me have a thorough understanding of 'how' we age, we don't have much knowledge on 'why.' It is currently beyond our scope of understanding. "Death" is also a questionable term, until you define "dead" and "alive." If you look at it from an obtusely technical standpoint, nothing ever "dies", since "living" things are simply large masses of atoms. "Death" and "life" is the association of properties given to certain "alive" bunches of atoms (such as the ability to move, think, grow, change, talk, etc...) that we, as humans, interpret to be the state of being "alive."
I'm sure others more in the know will give a scientific answer closer to the type of response you were looking for, but know that nothing really is true, it's just general accepted. In other words, there is no such thing as a 'objective' opinion, since we are all human. It's only relatively objective. And hence the necessity for humans to believe in greater beings and institute religions.