American Beauty is a profoundly sad movie. Or one could say, as does David Denby of The New Yorker, that it is "a funny movie that hurts."
He deliberately sets out to defy Platonic Truth, worshiping the Shadows—the felt, believed, imperfect, flawed perception of touched reality. This—and nothing else—is truth, or at least the only truth that matters.
It is no accident that "The English Patient"—surely the best movie to come on the American screen in a very long time—chooses the Greek Herodotus as a leitmotif.