As Elizabeth had no longer any interest of her own to pursue, she turned her attention almost entirely on her sister and Mr. Bingley; and the train of agreeable reflections which her observations gave birth to, made her perhaps almost as happy as Jane. She saw her in idea settled in that very house, in all the felicity which a marriage of true affection could bestow.
– Jane Austen
Pride and Prejudice, Chapter 18. Happiness can be infectious. Elizabeth turns her attention to how her sister Jane and Mr. Bingley interact at the Netherfield ball. The narrator describes how Elizabeth’s observations metaphorically give birth to happy thoughts about Jane’s romance blossoming into marriage. This makes Elizabeth almost as happy as Jane.