Because human being is not consist only of good things. Happiness, achievement, joy, laughter, friendship, love, health, and other gifts are all what our heart desires. But sometimes we need lessons and we know we deserve that. There are also sorrow, hollow, shameful, terrifying, sad, miserable feeling and it's a privilege to feel those too. It's probably a way to shape us into a more humble, softer, kinder, nicer, and wiser human being. So sadness, too, can make me smile.