Heights and wasps are the only "irrational" fears I have.
I don't fear mortality; my life is how I choose it. However, I often find myself in deep somber thought about my parents; they're older (dad is 61, mom is 58), and while they should have many years ahead of them, they're only human.
I suppose one could say I fear life without them and the companionship they provide me. It's hard not to take their presence for granted.