Why is Darcy worried about what Sir William says?
In Pride and Prejudice, the Lucases are good friends and neighbors of the Bennet family. We are told that Sir William Lucas was 'formerly in trade in Meryton,' and having achieved the distinction of being knighted, quickly removed his family to a house a mile from Meryton.
At a party in Netherfield, Elizabeth dances with Mr. Darcy, the man she hadn't originally meant to speak to at all. She is so surprised by his request to dance, however, that she agrees to go out on the floor with him. However, our heroine really hates Mr. Darcy's guts and they both trade barbs with each other after an initial, awkward silence. When Sir William passes by and comments on how well the two of them dance, he also hints at something which worries Darcy.
Allow me to say, however, that your fair partner does not disgrace you, and that I must hope to have this pleasure often repeated, especially when a certain desirable event, my dear Eliza (glancing at her sister and Bingley) shall take place. What congratulations will then flow in!
Mr. Darcy is not too happy at this hinting of a future marriage between his good friend, Mr. Bingley and Miss Jane Bennet, Elizabeth's sister. He later explains to Lizzy why he was so worried. Among other concerns, Darcy did not want his friend to be married to a woman with no connections whatsoever. He was also perturbed by the utter lack of propriety Lizzy's sisters, mother, and father often displayed. Last, but not least, Darcy felt pretty sure that Jane did not return Bingley's feelings for her; what's more, he claimed to have observed this with impartiality.
I did not believe her to be indifferent because I wished it; I believed it on impartial conviction, as truly as I wished it in reason.
So, you can see that Sir Williams' words really worried Darcy that his dear friend was entering into an unhappy alliance with an unworthy bride.