I subscribe to the Disposable Soma theory of aging. http://medicine.jrank.org/pages/1758/Theories-Biological-Agi... [1]

In this scheme cancer is just one example of the balance between somatic maintenance and reproduction rate.

[1] The disposable soma theory of aging predicts that species and cohorts in a population expecting, on average, to have high survival and low reproductive rates should invest more heavily in protecting their somas than species and populations that expect a short lifespan and to reproduce rapidly