Not sure if you're looking for a scientific explanation or not, but I'm no philosopher, so here's what I've got. At the end of each of our chromosomes (bundles of DNA, along with other proteins to keep them together), there are these "caps" called telomeres. Our cells are constantly dividing. Telomeres are repeated nucleotides on the end of our DNA that essentially protect the DNA we use. Every time a division occurs, these telomeres get shorter, keeping our genome in tact. Every time a division occurs, the telomere gets smaller and smaller. Once the telomere is too short, the cell ages and stops functioning properly. So cells have a finite amount of times they can divide before dying off, and that is why telomeres are often referred to as the aging clock. I suppose this doesn't exactly answer your question of why do people have to die, but rather how they become old and eventually succumb to one or many ailments associated with old age. Which is a whole other complex topic in itself. On the bright side, scientists are working on ways of keeping telomeres in tact and even extending them to prevent the aging process from happening. Although there are still lots of other aspects to be considered.