Elizabeth Bennett is not without her own fair share of pride, and Darcy's insensitive first marriage proposal really offended her.
In spite of her deeply-rooted dislike, she could not be insensible to the compliment of such a man's affection, and though her intentions did not vary for an instant, she was at first sorry for the pain he was to receive; till, roused to resentment by his subsequent language, she lost all compassion in anger." (Ch. 34)
Elizabeth, though incensed, still tries to be civil to Darcy, despite her feelings for the man, but at some point, enough is enough--she just could not listen to him abuse her family and low connections.
In the quote above, Elizabeth was just analyzing her mix of feelings on refusing Darcy. Of course, Darcy is utterly shocked that she would not consider his very eligible hand in marriage; he had entered the proposal fairly confident that she would say 'yes.' The following quote really sums up the extent of Elizabeth's feelings on the subject.
"From the very beginning— from the first moment, I may almost say— of my acquaintance with you, your manners, impressing me with the fullest belief of your arrogance, your conceit, and your selfish disdain of the feelings of others, were such as to form the groundwork of disapprobation on which succeeding events have built so immovable a dislike; and I had not known you a month before I felt that you were the last man in the world whom I could ever be prevailed on to marry." (Ch. 34)
Elizabeth, here, is extremely straight-forward with Darcy. He had wanted to know her reasons for refusal, so she lets him have it. Of all the sorts of responses Elizabeth might have given, Darcy was not expecting to hear her say this. In short, the quote really comes down to a personal attack on his character: selfishness, conceit, arrogance.