In Ch.53 Bingley returns to Netherfield along with Darcy. Soon, they visit the Bennets. Darcy is meeting Elizabeth for the first time after their surprise meeting at Pemberley. The situation is saturated with irony because only Elizabeth and the readers know of the contents of Mrs. Gardiner's letter [Ch.52] which testify to the fact that it was Darcy and not Mr. Gardiner who was responsible for saving the Bennet family honor by compelling Wickham to marry Lydia.
Darcy is a much changed man now. Similarly Elizabeth also has changed her opinion of Darcy after she has seen his altered behavior at Pemberley and after she has read Mrs. Gardiner's letter. On seeing him she expects him to warm up to her and show some sign of affection; but on the contrary he is his usual cold and reticent self. Not surprisingly Elizabeth is disappointed:"She was disappointed, and angry with herself for being so."
She tries to reason with herself and offers two probable explanations:
1. Mrs. Bennet presence:
He looked serious, as usual; and, she thought, more as he had been used to look in Hertfordshire than as she had seen him at Pemberley But, perhaps he could not in her mother's presence be what he was before her uncle and aunt. It was a painful, but not an improbable, conjecture.
Elizabeth knows fully well from Darcy's letter to her in Ch.35 how much he dislikes her mother, so she guesses that perhaps he is tongue tied in her presence so as not to give a reason for Mrs. Bennet to talk to him.
2. "He was not seated by her; perhaps that was the reason of his silence": Elizabeth reasons within herself that perhaps Darcy was silent because he was not seated next to him and moreover there were other people in the room.