Even as a kid, television shows and books taught me that it's good to be mortal. Immortality tends to revolve around having mortal friends who will one day perish and leave you feeling lonelier than ever (sure, better to have loved and lost but I can imagine it starts to drag after a time).
For example, much angst in Doctor Who is based around this idea that his life span is so much greater than that of his companions.
I'm pretty sure that way before I knew who Doctor Who was I already knew that immortality was a bad thing. Maybe I was just a really depressed kid. Maybe the circle of life song in the Lion King just hit home.
Perhaps it's knowing that searching for the fountain of youth is a wasted effort, but I think it's more that I don't *want* to live forever.
But why not? Why wouldn't I want to see what happens 100 years from now? Why wouldn't I want to witness the evolution of technology and mankind? Perhaps I'm afraid of having to witness the inevitable apocalypse (whether religious, through the death of the sun or alien invasion...), or maybe it really is just the whole 'oh woe is me all my friends are dead and I have too much knowledge in my brain.'
It's a clever trick, convincing a kid that immortality is bad so they shouldn't be afraid of their own mortality. Of course a child has little comprehension of what it means to be facing their mortality, but at the very least it seems possible for them to accept it's better than the alternative.
Again, logic and instinct don't tend to work in partnership, so I'm sure when the time comes for me to face it I'll probably do anything within my power to avoid it. Or, I'll avoid thinking about it.
Fugue state, please.