Reading this thread made me relook at Darcy's change of heart about Lizzy. And there isn't any particular logic to it.
Mr. Darcy had at first scarcely allowed her to be pretty; he had looked at her without admiration at the ball; and when they next met, he looked at her only to criticise. But no sooner had he made it clear to himself and his friends that she had hardly a good feature in her face, than he began to find it was rendered uncommonly intelligent by the beautiful expression of her dark eyes. To this discovery succeeded some others equally mortifying. Though he had detected with a critical eye more than one failure of perfect symmetry in her form, he was forced to acknowledge her figure to be light and pleasing; and in spite of his asserting that her manners were not those of the fashionable world, he was caught by their easy playfulness.
Mr. D didn't have a change of heart such as Mr. Tilney in Northanger Abbey, who we are told started loving Catherine because of her interest in him. There's no explanation given from JA at all about why, in particular. It just happened. So we can imagine as we chuse? ;)