Helmer's last words, which we are told are said when "a hope strikes him" in the stage directions, are shown to refer back to what Nora says just a few lines earlier:
HELMER: Nora--can I ever be anything but a stranger to you?
NORA: (picks up her bag) Oh, Torvald! Then the miracle of miracles would have to happen.
As Nora says, she and Helmer would have to change so much, and anyway, she doesn't believe in miracles any longer. Thus, the play rather cruelly ends with Helmer repeating this phrase with hope, when his words are cut short dramatically with the final sound of the play: the street door slamming shut and Nora leaving his life, asserting her own independence and determined to fashion her own identity. Helmer is left confronting the impossibility of changing himself to save his marriage, and the reality of how he has treated Nora during their marriage.