When I said Darcy was "falling" I did not mean he was falling in love...

Maybe I am not remembering correctly, but I seem to recall that very early in their acquaintance Darcy felt he was in concerned about his feelings for Lizzie-- at some of the early gatherings the narrator describes Darcy's perusal of Lizzie. And in Netherfield when Jane was sick, Darcy made a conscious effort to stay away from Lizzie the last day because he wanted to retain control of his emotions. I believe that Darcy was experiencing what most red-blooded males experience--physical attraction.

Clearly Darcy was conflicted. On the one hand he was attracted to Lizzie and found her more spirited and brighter than other ladies of his acquaintance, and the other hand he abhored her connections. She wasn't suitable for him as a wife, but he clearly was more attracted to her than he liked. (the narrator tells us this during Lizzie's stay in Netherfield). I wonder if he had met Lizzie in another setting, how he would have behaved? I don't know what it is, but Darcy appears to be a passionate man whose position in life forces him to keep everything in check. A bit of his sexuality came through when he observed Caroline Bingley and Elizabeth "taking a turn about the room" at Netherfield.

I think he "wanted" Elizabeth rather than "loved" Elizabeth, and I think he knew that this could not be. Hence his departure from Netherfield (we never really do know Darcy's point of view on this). And when he saw Elizabeth at Rosings, he realized his attraction for her still remained... He knew the only way he would get relief would be if he married her. I think when Darcy asks Elizabeth to marry her, it is not love he feels, but passion. It would have been a mistake if Eizabeth accepted him when he first proposed.

I also think that Elizabeth's refusal forces him to evaluate his emotions, and also makes Lizzie actually worthy for him to love. She refused him even when she knew of his position. His passion is now tempered with respect...

And to Adrian's point, I don't think someone as complacent as Jane could ever attract Darcy's attention the way Elizabeth did. You notice I said "strong minded beauty". I think Elizabeth's feistiness adds to her attractiveness and appeal in Darcy's mind.