Edward relates darkness with his life as a vampire. He has lived in the same body, at the same age, with the same family, for a hundred years. Time for him drags on, because he has lived all that time without someone to love. The part of him that regrets becoming a vampire is the part that hates the night, because that's the time when their true vampire nature is most prevalent (in the fact that they don't sleep like regular people, it's a good time for hunting, etc.). I think when Edward talks about the predictability of darkness, he's really explaining how his life has been a predictable routine for too many years; it is boring and tiring at the same time.