Darcy is an intelligent, somewhat cynical fellow, and I'm sure he sees right through Caroline's attempts to flatter him and befriend his sister. He is very nearly rude to her, but she doesn't take the hint. It's obvious Lizzy isn't even trying to make him think well of her, so he's intrigued. With his money, social position and good looks, he's probably never before met with a woman who didn't try to flirt or gain his good opinion.