TJD Prior writes to ask 'why do we die?', a question which has been an
important one in evolutionary biology for a long time. There have been
numerous possible answers - perhaps the most simply put is the essay by
P. Medawar in his collected works "The Uniqueness of the Individual".
Others answers have also been proposed, (e.g. the work of Mike Rose), but
Medawar's makes for good reading from a clever and agile mind.