He meant that American lives, or rather the American view of life, is all about the instant of success/failure and not about the full buildup and denouement, &c.
[editorial comment]
Personally I think Fitzgerald was a brilliant writer, but on this one he’s painting with too broad a brush. It’s true to some degree that we look for the one defining moment in a life, and then seem almost to resent people who insist on trying to continue on after some other shiny thing has grabbed our attention (Kato Kaelin, Scott “Wide Right” Norwood, John Rocker), but I think that’s mainly a function of mass media/pop culture, which is emphatically not a reflection of all Americans.
[/editorial comment]