We all have a basic psychological need to look after our minds and bodies. This is one of the prime directives of our unconscious minds and mentioned in the list of basic needs identified by the Human Givens Institute
It stands to reason that we have evolved this as a psychological driver. If we were not intrinsically designed to look after ourselves then surely, as a race, we could not survive. This drive exists across the natural world, every living thing lives with the overall drive to survive physically and mentally (within its capacity to do so).
What is amazing is the extent to which we, as humans, appear to push against this need!