The motivation of living creatures does got boil down to any single basic force, not even an 'instinct of self-preservation.' It is a complex pattern of separate elements, balanced roughly in the constitution of the species, but always liable to need adjusting. Creatures really have divergent and conflicting desires. Their distinct motives are not (usually) wishes for survival or for means to survival, but for various particular things to be done and obtained while surviving. And these can always conflict. Motivation is fundamentally plural... An obsessive creature dominated constantly by one kind of motive, would not survive.

Beast and Man: The Roots of Human Nature (1979).