When a composer has a great dance tune with no place to go, it's time to write a serenade. Serenades are on the populist end of classical music, which is why the San Francisco Symphony is devoting one of their summer pops concerts to them. Eine, kleine Nachtmusik is a case in point: We don't know why Mozart composed it, but since tunes dropped from his brain like water droplets off a wet collie, he didn't really need a reason, did he?