"Did not you think, Mr. Darcy, that I expressed myself uncommonly well just now, when I was teazing Colonel Forster to give us a ball at Meryton?"

"With great energy; but it is a subject which always makes a lady energetic."
"You are severe on us." (6)

Thank you Adrian! (:D) I think Darcy might have expected an energetic response from Lizzy in asking about a reel because it is likely he did not realize how his response (above) could appear condescending. He seems to be generalizing that all ladies are rapturous on the subject of balls therefore Lizzy’s expressing herself with great energy is rather common and no surprise to him in the least. Oh dear! This interpretation would only enhance her belief he views her as an inferior and a silly one as well. Did Darcy mean to be teasing or was he expressing a belief? Which ever it might be it appears Lizzy thought he was being severe on ladies. Lizzy goes on to say she understands she is not up to his standards on the pianoforte, “I would really rather not sit down before those who must be in the habit of hearing the very best performers” and unable to escape exhibition, “gravely glancing at Mr. Darcy, "There is a fine old saying… 'Keep your breath to cool your porridge' -- and I shall keep mine to swell my song” (6). Perhaps in this last Lizzy is saying my breath is better spent in song than in talking to you! If this or something like it was Lizzy’s views in Ch. 6 it is no wonder she immediately suspects Darcy’s “feel like dancing a reel” question (10) to be an attempt to trick her into exposing herself:

"Do not you feel a great inclination, Miss Bennet, to seize such an opportunity of dancing a reel?"

I guess Lizzy hears something like this: “At hearing a note of lively music, being a female, you must be chomping at the bit to dance.”

She smiled, but made no answer. He repeated the question, with some surprise at her silence.

When she does not attack the subject of dancing right off, Darcy as if he thought she would, is surprised and asks her again. Perhaps asking again only reinforces the idea (to her) that he believes she is a silly creature quick to rapture and enthusiasm at the thought of dancing a reel. Of course even if she felt this way she would not give Darcy the satisfaction of knowing it or using her reaction for sport:

"Oh!" said she, "I heard you before, but I could not immediately determine what to say in reply. You wanted me, I know, to say 'Yes,' that you might have the pleasure of despising my taste; but I always delight in overthrowing those kind of schemes, and cheating a person of their premeditated contempt. I have, therefore, made up my mind to tell you, that I do not want to dance a reel at all -- and now despise me if you dare."

"Indeed I do not dare."

[Lizzy’s view:] Elizabeth, having rather expected to affront him, was amazed at his gallantry; [Narrator’s view of Lizzy:] but there was a mixture of sweetness and archness in her manner which made it difficult for her to affront anybody, [Darcy’s view of Lizzy] and Darcy had never been so bewitched by any woman as he was by her. He really believed that, were it not for the inferiority of her connexions, he should be in some danger. (Ch. 10)