What could possibly make Lady Catherine deliberately throw Elizabeth and Darcy together?

Elizabeth Bennet cannot believe the effrontery of Mr. Darcy's proposal to her, but before she can firmly decide to despise him, he hands her a letter. Unsure what he could possibly have to say to redeem himself, she sneaks off to read his words, only to witness the flight of Miss Anne de Bourgh. Now, despite the array of turbulent emotions between them, Lady Catherine de Bourgh insists that Elizabeth and Mr. Darcy form a team to search for her daughter. Though Lady Catherine has motives of her own, being thrown together may help Mr. Darcy and Elizabeth better understand each other, all while they go after Anne.