Dang, why are you asking such a deep question, Jinie. But in my own perspective, happiness exists without sadness is possible, as we can seek the simplest happiness. Like, when you found money in your pocket, or when you finally got the chance to have a proper me-time. Those don't need sadness to happen. It depends on how we put the value and meaning of the 'happiness' itself. All above is about mindset.