Because it's just like. Darcy did everything Lizzy ever told him to, yknow? He asked how she thought people should kindle affection and she said dancing, so then he asked her to dance. She asked what he qualifies as an accomplished woman, he looked at her and the book she was reading, and said a well read woman. She said he should talk while they're dancing, so he asked about her and her sisters going to Merryton. She told him he should practice talking to people he's not already familiar with, so he came over and visited her the very next day. You know??? Do you see???? He has been shouting from the very start that he cares for her, in his own way, just as Jane shouted her love to Bingley, in her own way. And then when she was visiting Rosings and Lady Catherine de Bourgh, his own aunt, was being obnoxious and rude to her, he felt the shame of having undesirable connections just as she had in the beginning. Do you see it!!!! Mr Darcy has been ardently in love with her from the very beginning, and he's been missing the parallels between their lives the whole time!!! He's so pathetic and sopping wet it hurts!!!