I love the feel of hitting the ball hard, the pleasure of a rally. It is these things that make tennis the delightful game that it is.
But I think it's more that when you're young, you're invincible, you're immortal - or at least you think you are. The possibilities are limitless, you're inventing the future. Then you get older and suddenly you have a history. It's fixed. You can't change anything. I find that a bit disturbing, to be honest.