Declan Burke asks, "[A]ny theories as to why crime fiction takes such a compelling grip as to last you an entire lifetime?"
My comment, with some additional thoughts:
I was fascinated with movie and TV detectives at a young age, but as many have said, I began to read my favorite characters in my late teens.
I don't need to to see good beat evil or order restored from chaos, but I do enjoy a sense of "roundness". Events or details from the beginning of a story play into the end. I enjoy trying to solve puzzles along with the protagonist (as opposed to waiting for the protag to make brilliant, surprising deductions). This is a level of reader participation found in no other kind of story.
Lastly, crime fiction seems to present worlds I can most easily believe. Sure, they may be too dark for some, but I prefer a little dark to too light or far-fetched.