"And that is quite impossible; for he is now in the custody of his friend, and Mr. Darcy would no more suffer him to call on Jane in such a part of London! My dear aunt, how could you think of it? Mr. Darcy may, perhaps, have heard of such a place as Gracechurch Street, but he would hardly think a month's ablution enough to cleanse him from its impurities, were he once to enter it; and, depend upon it, Mr. Bingley never stirs without him." (Chapter 25)

Darcy’s feelings change for Lizzy from attraction to real emotion during his visit to Rosings. Why does Lizzy not realize this? I think there are several reasons. She believes Wickham’s story and that Darcy is not a good man—Chapter 16. She believes he helps the Superior Sisters keep Bingley from Jane—Chapter 25. Lizzy has never understood that Darcy’s appraisal of her had changed since Chapter 3 so she often suspects his motives when interacting with her—she has often misinterpreted his attention to her as satirical rather than sincere. She looks for other motives rather than what might seem apparent. I thought a look at these misinterpretations up to the proposal would be interesting. ;D

Colonel Fitzwilliam entered into conversation directly with the readiness and ease of a well-bred man, and talked very pleasantly; but his cousin, after having addressed a slight observation on the house and garden to Mrs. Collins, sat for some time without speaking to anybody. At length, however, his civility was so far awakened as to inquire of Elizabeth after the health of her family. She answered him in the usual way, and after a moment's pause, added -- (Chapter 30)

Darcy visits the parsonage the day after his arrival and Charlotte attributes the quickness of the visit to Lizzy’s presence: "I may thank you, Eliza, for this piece of civility. Mr. Darcy would never have come so soon to wait upon me." Lizzy remains insensible of the compliment however greeting Darcy coldly and then asks if he saw Jane in town well aware that he did not only hoping to see if he shows any signs of guilt about Jane. During the visit Col Fitzwilliam endears himself to the ladies with his gentlemanlike manners and Darcy sits for some time without speaking much as he often did at Meryton. Silent staring has yet to impress Lizzy; she likes an open manner and temper like Wickham and Col Fitzwilliam which is also like her own. Her history with Darcy, his looking only to criticize (Chapter 6) of which she well aware still holds sway over her interpretation of his behavior. He continues staring and speaking little, being outshone by his cousin.

"You mean to frighten me, Mr. Darcy, by coming in all this state to hear me? But I will not be alarmed though your sister does play so well. There is a stubbornness about me that never can bear to be frightened at the will of others. My courage always rises with every attempt to intimidate me." (Chapter 31)

Darcy continues his policy of observing Lizzy without speaking until he approaches her and his cousin at the pianoforte in Chapter 31. Lizzy suggests he intends to frighten her which I think means she believes he comes to criticize her playing. It starts as soon as he moves towards her. Lizzy goes on to criticize Darcy IMO by suggesting he could recommend himself to strangers if he practiced. She thought Darcy came to criticize and she rose to the challenge and criticized him first.

As he spoke there was a sort of smile which Elizabeth fancied she understood; he must be supposing her to be thinking of Jane and Netherfield, and she blushed as she answered… (Chapter 32)

IMO Darcy brings up the “good roads” between Hunsford and Meryton in relation to Charlotte to sneakily gain information about Lizzy’s feelings on living far from home. It completely backfires as Lizzy gets embarrassed because she thinks Darcy might believe she is speaking of Jane and Netherfield—suggesting a match between them. Lizzy is particularly conscious of anything which might make her appear to be chasing after Bingley for Jane or Darcy for herself. She was extremely embarrassed when Darcy and many others overheard her mother talking of a Bingley and Jane match at the Netherfield ball.

It seemed like wilful ill-nature, or a voluntary penance, for on these occasions it was not merely a few formal enquiries and an awkward pause and then away, but he actually thought it necessary to turn back and walk with her. He never said a great deal, nor did she give herself the trouble of talking or of listening much; but it struck her in the course of their third rencontre that he was asking some odd unconnected questions -- (Chapter 33)

In Chapter 33 Lizzy has told Darcy where her favorite walks are in the park so he can avoid meeting her and incredibly he continues to meet her there. This seems so pointed an attention that it is at first hard to see why Lizzy fails to see it but it goes to show that her history with Darcy is incredibly difficult to overcome. I think it also shows that once you are not believed to act like or have the proper sentiments of a gentleman it takes a lot to prove otherwise. His past behavior towards her has proven that he does not like her and only looks to criticize so his odd questions could only have to do with her and Col Fitzwilliam. Col Fitzwilliam’s information about separating Bingley from Jane in Chapter 33 is a confirmation, once again, of Darcy’s dishonorable nature, Wickham’s story and kills any surviving good will she may have had for him, and finally even believing he boasted of it to Col Fitzwilliam, Chapter 34: Mr. Darcy's shameful boast of what misery he had been able to inflict gave her a keener sense of her sister's sufferings. Darcy has no chance with Lizzy after this.

But why Mr. Darcy came so often to the Parsonage it was more difficult to understand. It could not be for society, as he frequently sat there ten minutes together without opening his lips; and when he did speak, it seemed the effect of necessity rather than of choice -- a sacrifice to propriety, not a pleasure to himself. He seldom appeared really animated…He certainly looked at her friend a great deal, but the expression of that look was disputable. It was an earnest, steadfast gaze, but she often doubted whether there were much admiration in it, and sometimes it seemed nothing but absence of mind. (Chapter 32)

IMO if Charlotte cannot determine a little of Darcy’s feelings and thinks he is not enjoying himself then it would be impossible for Lizzy to do so with their history. Over and over again Lizzy is annoyed that Darcy does not help with conservation—Chapter 32, “Elizabeth made no answer. She was afraid of talking longer of his friend; and, having nothing else to say, was now determined to leave the trouble of finding a subject to him.” Darcy seems to visit only to be an uncomfortable presence. I think his silence, his reserve is a habit which Lizzy finds particularly ungentlemanly and it adds again and again to her conviction that he is above his company and on occasion that he finds something wrong with her. His veiled roundabout way of collecting information about her also does not help relieve this impression or that of his looking only to criticize. His eavesdropping on her conservations, watching her without speaking and his trying to find out her feelings by talking around a subject has only made her believe worse and worse of him. ;D