You change. You gain perspective. You focus on different bits. And you miss things the first time round. Take the Iliad. A young kid might love the gory tongue-splitting spear through the head of the enemy fighting scenes of the Iliad. Later on in life you might dwell on the fact that a good chunk of the story is dedicated to humanizing Hector, the hero of the enemy. Showing us his concern about the fate of his wife and children if Troy does, in the end, get sacked. then again, i can't get my wife to watch or read something twice ever.