At fifteen, he'd still probably been working through his cabal trauma and the knowledge that a bunch of his family had voted for him to go back or something. Regardless of that, though, the realization had hit! Down with Harriet!

Also, while the narrative would never condone such behavior, imagine how hilarious it would have been to trade Caroline Bingley's dancing skills. Not that Lydia would do that any more, surely.

"And I don't think we have anything quite like that," he mused. "Being able to actually trade someone's skill for our own. Hmm." It wasn't his place to chastise Lydia and besides, it sounded like the kind of thing she'd grown out of.