Even Darcy himself struggles to answer this question. In chapter 60, Elizabeth asks Darcy to explain how, when, or why he fell in love with her:
she wanted Mr. Darcy to account for his having ever fallen in love with her. “How could you begin? . . . what could set you off in the first place?”
At this point in the novel, Elizabeth has already agreed to marry Mr. Darcy. Neither Elizabeth nor Mr. Darcy greatly enjoyed each other's company early in the book. Mr. Darcy, however, steadily grows in admiration of Elizabeth. He proposes to her, for the first time, midway through the novel. She refuses this first proposal, shocked that he would ever consider such a match, but she learns to appreciate and love Mr. Darcy as they continue to interact in the second half of the book. He responds to Elizabeth's question:
I cannot fix on the hour, or the spot, or the look, or the words, which laid the foundation. It is too long ago. I was in the middle before I knew that I had begun.
She then recounts their initial observations of one another and early feelings toward each other. At first, he had refused an opportunity to dance with her with his infamous line,
She is tolerable, but not handsome enough to tempt me (chapter 3).
Clearly, Darcy does not fall in love with Elizabeth simply for her appearance. She also doubts that he could have fallen in love with her early manners and actions:
my behaviour to you was at least always bordering on the uncivil, and I never spoke to you without rather wishing to give you pain than not. Now be sincere; did you admire me for my impertinence? (chapter 60)
Impertinence refers to disrespect or discourtesy. In our modern language, we might call impertinence an extreme form of sassiness. Darcy takes this negative word, impertinence, and rephrases it in a positive manner:
For the liveliness of your mind, I did (chapter 60).
He tells her that he appreciated her clever words (or, perhaps, her well-planned sass). Additionally, he mentions the "affectionate behavior" that Elizabeth showed Jane when she got ill and was forced to stay at the Bingley's house until she got better. Mr. Darcy learned to look past mere physical appearance. Yes, it does seem that Mr. Darcy grew to think that Elizabeth is beautiful as time progressed. In a conversation with Miss Bingley he announces,
I have been meditating on the very great pleasure which a pair of fine eyes in the face of a pretty woman can bestow.
When she asks him whose eyes captured his attention, he responds:
Miss Elizabeth Bennet (chapter 6).
However, it isn't Elizabeth's physical beauty alone that draws his love and affection. Darcy admits to appreciating Elizabeth's active mind and her passionate concern for her friends and family. Her love and concern for others is seen not only when Elizabeth cares for her sister Jane when she got ill, but is also seen when Elizabeth goes to visit her friend Charlotte Lucas after Charlotte marries an irksome husband for money and comfort rather than love.