To sing the Great American Songbook convincingly, it helps to believe in chance. All the legendary composers of standards - George and Ira Gershwin, Richard Rodgers and Lorenz Hart, Harold Arlen, the list goes on - had something to say about life's serendipities. Their songs are full of unexpected encounters, fine romances that blossom out of nowhere. Their wistful and often impossibly beautiful melodies convey the magic of happenstance and also its flipside, the capricious cruelty of fate. Their lyrics celebrate the notion that life can change in an instant - when that vision of loveliness steps out of a dream and you suddenly find yourself bewitched, bothered and bewildered.See Boz Scaggs perform at Humphreys Concerts by the Bay, on June 7 at 7:30 p.m. .