I believe the explanation for Lizzy's smile can be found in the text: They stood for some time without speaking a word; and she began to imagine that their silence was to last through the two dances, and at first was resolved not to break it; till suddenly fancying that it would be the greater punishment to her partner to oblige him to talk, she made some slight observation on the dance. In the novel, the initial thought and the action are in the same sentence.