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2. We represent one of the current twigs on the bush of evolutionary development: our minds have evolved as much as the rest of our bodies, and we have inherited our ways of thinking/feeling/reacting from ancestral species, modified over time.
3. It seems reasonable to argue that a relative willingness to suicide, whether by actively and fatally injuring oneself or by refusing to eat or drink, would be a competitive disadvantage in an ancestral species. Not matter how bleak the outlook might be, in the life of an animal still capable of reproduction, some part of the time, that animal will survive and reproduce... unless it suicided. So the animals that tend to suicide will reproduce less frequently than those that don't.. all else being equal. Thus natural selection will tend to produce animals that cling to life rather than animals that give up easily.
4. Fear of death is a reasonable corollary of the will to live... and fear of death will be a competitive advantage. Any animal that is indifferent to risk of death or serious injury (serious injury usually means death to most animals and humans before modern medicine) will expose itself to more risk of death than would the one afraid of the risk.. and therefore will die younger, and leave fewer descendants.
5. Fear of death may also have played a role in the development of religion, altho Diamond in Guns, Germs and Steel (and others) have attributed much of the success of organized religion to efforts by early leaders of human societies to maximize their power over their subjects.
So I take issue with the introductory assumption, in the OP, that living or dying is a matter of choice. It is, in the sense that we all have the ability to kill ourselves, but it isn't, in the sense that our minds allow us to select either 'choice' with no preset (and extremely powerful) bias against the death option.
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