Scientists are very interested in your question. They're studying what causes our cells to die. Their name for cell death is 'apoptosis.' You could search for that on www.google.com. A big question in science is whether we could live to be very old if we learned how to control the things that kill our cells. There's also some interesting research on why people and animals seem to sometimes live longer if they don't eat very much. There seems to be a vitamin that we use for digesting food that can also protect our cells from dying, but if all gets used up in digesting our food, there's not so much for keeping our cells alive. This research is very new, so it may turn out to be much more complicated than that, but it is quite exciting.