Is There a Difference Between Happiness and Sadness?

i don't quite think that sadness is the opposite of happiness. Sometimes i think it's helpful to feel sadness in fact, like i used to be on a drug for my o c d which caused me not to really be able to feel sad. Ah, and it felt like there was a real loss in not having that. But i will say, i mean, when i wrote the fault in our stars, i thought that  it would be a book that would be read by nerd fightersand people who knew or liked my previous work. By it never crossed my mind that it would be so popular, just because it seemed to me so sad.