One can remember that it is at Rosings that Mr Darcy aknowledges he cannot live without Elizabeth and decides to marry her.
And maybe, knowing that she wanted so much her nephew to marry his cousin Anne de Bourgh, and didn't conceal it, was it easier for him to reject all the women who were introduced to him in the hope of marrying one of them.
Would Lady Catherine be, in fact, a Cupid? Miss Austen, you are teasing us!
(link : Youtube extract of Don Quixotte ballet's Cupid variation, by Nina Kaptsova)