First, I have a confession to make: Darcy  did not immediately win me over when I first read Pride and  Prejudice as a teenager. His first proposal was disastrous and  though his second was better, there just didn't seem to be much to swoon  over. I was much more taken by Elizabeth's strength and wit. I wanted  to be Elizabeth, rather than be with Darcy. 
That didn't really change until I decided  to tell Darcy's story. To gain a better understanding of him, I  read the novel once more with I had two questions in mind: 1)  What made  him so reluctant to give his good opinion to others, and 2)  How did  Elizabeth so easily gain it, almost against his own will?
 The  answer completely won me over.
 
Darcy prizes honesty above all else. As a  man of position and wealth, he is accustomed to  being used and pursued. Men want to be known as his friend and women  want to be courted by him. His disdain of this is clear in his response  to Miss Bingley in Chapter Eight: "Undoubtedly," replied Darcy, to whom  this remark was chiefly addressed, "there is a meanness in all  the arts which ladies sometimes condescend to employ for captivation.  Whatever bears affinity to cunning is despicable." Caroline catches  enough of his point to drop the subject.
Elizabeth never fawns over Darcy, and thus he cannot help but fall in love  with her. Unfortunately, it does not occur to him that if she shows him  the barest civility, she actually might not like him. This he discovers  in his proposal, when that same admirable honest streak leads him to say  things he perhaps ought to have kept to himself.
However, Darcy is not just honest with others, he is also honest with  himself. After the immediate sting of Elizabeth's rebuke dulls, he sees  the truth in her words and he resolves to change. He respects her  opinion enough to trust her insight, even when her words hurt.
An honest man who wants nothing more than to be deserving of an honest  woman? 
Swoon. Then he takes Lydia's rescue upon himself to save  Elizabeth the pain of having a fallen woman for a sister and his  character is fixed as the noblest man in Derbyshire. I could not help  but fall in love with him, as generations of women before me have done.
When I started writing 
His Good Opinion, that honesty and disdain of falsehood drove the opening chapter of the story. Listen to the conversation between Bingley and Darcy:
"I  will never understand, Darcy, why you insist on going out in Society  only to be displeased with everyone you meet."
Fitzwilliam Darcy poured two glasses of brandy and handed one to his  friend before he took the chair opposite him. "I go out because it is  expected of me, Bingley. You know that."
Charles Bingley pointed at him. "Ah, but that does not answer the  question, does it?"
Darcy conceded the point with the barest shrug of his shoulders. Here,  in the comfort of his own study, there was no need to pretend. "I admit  that I find little in Society of which to approve."
"Only because you are determined to disapprove!" Bingley protested.  "What of the young lady you sat out with tonight? Let me hear your  opinion of her."
Darcy ran his fingers down the side of his glass. "Her aunt approached  me and said her niece had sprained her ankle, and would I be willing to  keep her company? Courtesy forbade I refuse, though you know how little I  enjoy making conversation with someone I am not intimately acquainted  with. I have not your ease of speaking on subjects in which I have  little or no interest." His lips curled in disdain, and he took a sip of  brandy to wash the sour taste from his mouth.
"That is a commentary on your own character, not the lady's."
He ignored the familiar needling. "After two minutes of idle chatter, I  inquired after her injury."
Satisfaction gleamed in Bingley's blue eyes. "Ah, you are capable  courtesy after all!"
Darcy leaned forward, his forehead creased in a frown. "Perhaps you will  not be so victorious, Bingley, when you hear the rest of the story. She  did not understand what I spoke of. When she returned to her aunt  shortly thereafter, she did not have a limp. The entire incident was  manufactured so she could gain my attention. No doubt they have heard  that I do not dance often —"
"Or ever."
The leather chair creaked in protest when Darcy stood. He took Bingley's  glass and strode to the table, glad to have something to do, even if it  was only refilling their drinks. This topic never failed to rile him,  and he could not sit still. He poured the amber liquid and found a  measure of calm in the action.
"They sought a way to get time with me, and they found it. You wish to  know why I so seldom give my good opinion to those I meet; it is this  dishonesty, this deception of which I cannot approve. I cannot — I will  not — marry a woman I do not trust."
And there you have it: Fitzwilliam Darcy, an honest gentleman searching for an honest lady. Might I volunteer, sir?