Happiness is a warm, exhilarating feeling. And being happy because of someone else is even more thrilling: a lightness like you're a balloon buoyed up with air. It's out of your control -- which can be scary and leave you reeling, or amplify your happiness beyond a factor you yourself could achieve.

Being happy like that feels like driving at night up a long stretch of road and hitting every green light, one after another after another. Like walking into the subway station just as the train pulls up. It's Neil Diamond's "Sweet Caroline" and Sam Cooke's "Wonderful World" and Bruce Springsteen's "Dancing in the Dark" coming on the radio. Flipping your pillow onto the cool side in the middle of the night. A drink of cold water when you're incredibly thirsty.