I think stewartkv summed it up pretty well. When you move up it makes you more aware of the game and what you are doing well or not so well.

Due to me cutting my own BR in half from a withdrawl, I recently moved down to $25 no limit where I am destroying the game. I think I might have played to long at the $50 nl where it got to be monotonous and I made all the standard raises and calls without really thinking about the game.