maybe it's the concept of the complete and final unknown. When I think about I've been alive for my entire life so it's hard to imagine not existing - having no consciousness. Like I've said before, when I die I'll go back to doing what I did before I was born - a whole lot of nothing.
“I do not fear death. I had been dead for billions and billions of years before I was born, and had not suffered the slightest inconvenience from it.”
Mark Twain