You have to experience sadness to know happiness

"Some days are just bad days, that's all. You have to experience sadness to know happiness, and I remind myself that not every day is going to be a good day, that's just the way it is!" -Dita von Teese

Just like you have to experience cold weather to know hot weather, you have to experience knowing before you feel confused, you're feelings are hurt before you learn to stop hurting others' feelings, you have to work hard before you can feel relaxation, you have to give up once before you learn when to keep going, you have to lose before you can win, you have to sink before you can float.

For every action, there is an equal and opposite reaction. Perhaps Newton's Third Law applies beyond physics.