Boredom 
I really wonder how the story would have gone if Ryuk hadn't been bored. Because if he hadn't, then he wouldn't have dropped the Death Note, which wouldn't have caused Light to pick it up, because there would be no dropped Death Note in the first place. (People say I think too much.)
Screenshot is from here:
http://animestocks.com/gallery/category/Death+Note/Ryuk
