Why does Darcy’s first proposal go so badly?

Darcy proposes while still convinced that loving Elizabeth is a concession. He dwells on her family’s impropriety and inferiority even as he declares passion — so the speech insults the person it claims to honor. Elizabeth hears the class contempt more loudly than the love, and she already believes he ruined Jane’s happiness and wronged Wickham.

The disaster is structural, not just awkward wording. At Hunsford he has not yet done the moral work the letter and later actions will force: separating genuine faults in the Bennets’ behavior from the pride that made him a bad witness of his own motives. The failed proposal is therefore evidence for “first impressions” and “self-knowledge” at once — both characters are still defending a story about the other that the novel is about to break.