That's not THE definition of happiness. That's HIS definition of happiness. I've read easily over half a dozen books on happiness -- from a Buddhist point of view, from a Christian point of view, from an atheist point of view, from a neuroscientist point of view, from a historical point of view and from a experimental point of view. None of those books defined happiness as constant joy. That would be highly unlikely to maintain. It would be too exhausting. His source of inspiration is also a little dubious. He cites Augusten Burroughs as his source. Augusten Burroughs wrote a memoir about being abandoned by his psychotic mother and fostered by a crazy psychiatrist, looking at the Amazon review. I wouldn't take my definition of happiness based on that one person.