This one always makes me pause. I remember first getting into drawing skulls and skeletons in high school. I once asked a guest psychology teacher in my English class what they meant and he said it was generally considered to be a fear of the future. At the time, what he said didn't mean much to me. Over the years I've come to realize that there's a lot of truth to that answer.
Honestly, though, it is a bit more complicated than that. I've always loved books about the human body, skulls and skeletons have always been symbols of hardcore coolness and rebellion to me and the best test score I ever got in Advanced Biology in high school was the human skeleton test. Feh, it's never simple, is it?