well it's dark, alone in the dark, That book, y'know, the old picturew book where it was all like "{In a dark, dark house on a dark, dark hill", NIGHT of the living dead. It is kind of associated with bad things. Though I think people become afraid of the dark mainly because at a young age kids are taught that y'know, monsters live in the dark, the buy you nightlights and treat them as a symbol of protection, protection against THE DARK (Y'know, I didn't have a night-light). And I think they do that because they just know that if all kids weren't afraid of the dark then eventually SOMEBODY would trip on something they couldn't see and fall into the nearby woodchipper which was accidentally turned on when someone bumped into it.
Besides, I have a pretty wild imagination, right now I've just got this little lamp right next to my computer, the rest of this room is covered in horrible, horrible darkness, like the very essence of death.