I came to another important conclusion about defining happiness: that the opposite of happiness is unhappiness, not depression. Depression, a grave condition that deserves urgent attention, occupies its own category apart from happiness and unhappiness. Addressing its causes and remedies was far beyond the scope of my happiness project. But even though I wasn’t depressed and I wasn’t going to attempt to deal with depression in my framework, there remained much ground to cover—just because I wasn’t depressed didn’t mean that I couldn’t benefit from trying to be happier.
— Gretchen Rubin, The Happiness Project: Or, Why I Spent a Year Trying to Sing in the Morning, Clean My Closets, Fight Right, Read Aristotle, and Generally Have More Fun.