Over on Reddit a guy with terminal cancer posted that he was going to die in 51 hours and wanted to spend some of this time talking about it with Reddit users. Lots of interesting stuff in the thread. (BTW, it was a hoax. No matter, the ideas are valuable anyway.)
Someone asked if he was scared of dying. He said he was terrified. What a word. From there the discussion goes to the usual places, the things we have all thought of. But there's a new idea I have, at least for me, that I've never written about. So I thought I should.
Every animal has the instinct to fight death. I suppose there are occasional mutations, individuals that don't care whether they live or die, but they would get culled quickly. The more fanatic you are about not dying the more likely you are to reproduce. Evolution reinforces this trait.
I think that's where the terror comes from. If we lived in a totally benign environment, where our time of death was fixed, and nothing could change it, not even our disposition, we would never have developed the terror.