I personally feel that this quote is lost in translation. The Greek word for “happiness” (εὐδαιμονία) literally means “good spirit” (eu + daimon), and can be more accurately translated as “human flourishing”. I think this is more analogous to joy, or “ the way” in Taoism, of which both have no opposite. The polar opposite of “happiness” on the other hand is “sadness” which makes it a very fickle concept. However, I do not think Aristotle was talking of “happiness” in this sense.