You get sad for almost the same reason you get happy: something happens and you let it fill you up and overwhelm you. If happiness and sadness were colors, they would be yellow and blue respectively. When you mix yellow and blue, you get green. Green means growth, newness, a fresh start. So all you really have to do is figure out the right combination of yellow and blue and figure out how to balance your happiness and sadness, and you can start feeling okay again.