In Volume 2, Chapter 10, Elizabeth tells Colonel Fitzwilliam, "Mr. Darcy is uncommonly kind to Mr. Bingley, and takes a prodigious deal of care of him." Colonel Fitzwilliam then reveals to Elizabeth, "Care of him! - Yes, I really believe Darcy does take care of him in those points where he most wants care. From something that he told me in our journey hither, I have reason to think Bingley very much indebted to him. But I ought to beg his pardon, for I have no right to suppose that Bingley was the person meant. It was all conjecture." Then Fitzwilliam continues, "And remember that I have not much reason for supposing it to be Bingley. What he told me was merely this; that he congratulated himself on having saved a friend from the inconveniences of a most imprudent marriage, but without mentioning names or any other particulars, and I only suspected it to be Bingley..." Fitzwilliam also told Elizabeth, "I understood that there were some very strong objections against the lady."
Elizabeth is very upset with this news and the possibility that Darcy had ruined her sister's chance for happiness. Even though Elizabeth wanted to think positive that Darcy didn't interfere, she couldn't understand why anyone would have any objections against Jane because she was so sweet, loving, and thoughtful.
Reference: Thornes Classic Novels: Pride and Prejudice by Jane Austen. Stanley Thornes Publishers, England. 1997.
Colonel Fitzwilliam reveals that Darcy saved Bingley from what would have been an "imprudent" marriage . . . and Elizabeth is able to put that in context. She realizes that her beloved sister, Jane, was the unworthy potential bride. "I understand there were some strong objections against the lady," Fitzwilliam says, and Elizabeth is infuriated; Darcy has been the cause of her sister's misery all along.