Against a short serve, you can take the ball quick and rush the opponent, you can go for angles, and you can drop the ball short. So have a number of ways to mess up an opponent without actually attacking the serve. This is where you can get really creative. Against a deep serve, you don't have these options. You can't rush the opponent with a quick shot, go for extreme angles, and you can't return it short. If you return the deep serve passively, you are giving your opponent lots of time to set up his best shot. So don't.