You should read this
A Dialogue on Happiness - Existential Comics
I just think it is a really cool dialogue on happiness. I still believe what I said though. I don't think we need sadness for there to be happiness, but I think that without sadness of any sort one can't truly be happy. Yes, the emotional state happy, but I don't think you could be thoroughly satisfied and happy with yourself on all mental levels without sadness. I think you might feel happy, but you would still feel incomplete at the same time.