Ask most people why we die as we grow old, the answer you most likely will get is that our body slowly wears out, getting weaker and weaker with increasingly delicate immune system that is no longer able to fight disease or physical injury. But that really begs the question: why is it that we get weak as we get older? The mechanistic answer that body parts wear out is not satisfactory because the body parts don't have to wear out. After all, we do in fact get stronger between infancy and early adulthood.
The body is not like a machine with inert parts. We consume food and water and our cells grow and repair themselves. In other words, evolution could have easily decided that such repair and resuscitation is necessary and kept our bodies with equal strength and vigor all our lives. We would still die because our resources to fight disease, predation and other physical strengths are not infinite and every now and then we would succumb to an external threat of predation, disease or accident. But we would stay young, beautiful and strong all our lives and a multi-billion dollar industry hopelessly attempting to help people fight aging and senescence would disappear.
But evolution chose not to arrange our lives that way. Why not? The answer, it turns out, is that because resources necessary to fight external threats are limited, it is more important to allocate more resources to the body when it is young than when it is old. This is because death is irreversible. If you die while young, you wouldn't be around to become old. On the contrary, if you die when you are old, that is not as bad because you still enjoyed being alive (which means procreate passing your genes to the next generation) while young. Thus genetic mutations that allocated resources equally, keeping us equally strong all our lives, will lose the evolutionary race to genes that front-load the resources early on. This argument was articulated nearly 50 years ago by biologist George Williams in a paper he published in 1957. He wrote:
...natural selection will frequently maximize vigor in youth at the expense of vigor later on and thereby produce a declining vigor (senescence) during adult life.
I developed a mathematical model to capture this insight and to explore what else would such evolutionary optimization imply. In a paper that is forthcoming in the journal Economic Inquiry, I show that not only does the possibility of death imply that organisms value the future less, compared to the present (called discounting in Finance and Economics), it also makes sense to avoid risk in the sense organisms would tend to avoid gambles that have equal probabilities of increasing resources and decreasing resources by an equal amount (called fair lotteries). The intuition is that a decrease in resources has a disproportionately larger negative impact compared to an equal amount of increase in resources because the decrease is more likely to cause the organism to die and thereby permanently forgo the possibility of producing offspring in the future that will contribute to its genetic fitness. So, possibility of death causes us to value the future less, makes us weaker as we grow old, and predisposes us to avoid risk, following a simple evolutionary argument. You can read the paper with the mathematical model here.