Some time ago I wrote an article about one of the biological reasons why we have to die.

There I also pointed out that the concept to die and give ones genes to the next generation most of the time seems to be superior in evolution compared to the concept of immortality (which actually exists as well). The reason for that is that it allows to adapt more easily to a changing environment according to the motto "we age because the world changes" - also computer simulations seem to confirm that.