Mr. Darcy undergoes something of a personal transformation over the course of Pride and Prejudice. When we're first introduced to him, he comes across as arrogant, aloof, and pumped full of pride. That's certainly how Elizabeth Bennet regards him. She soon develops a deep-seated prejudice towards Darcy that thwarts any chance of a deeper relationship between them.
Over the course of the book, however, both Darcy and Lizzie change dramatically. Lizzie loses her prejudice towards Darcy when she realizes what kind of a person he really is; and Darcy stands revealed as a kind, humble person whose previous arrogance was really just a protective shield.
By the end of the story, Darcy has learned to be more humble, better able to connect with other people instead of keeping them at arm's length. This enables him to express the true depths of his feelings towards Lizzie, which, duly reciprocated, lead to their getting hitched.
One certainly can't imagine the icy, superior figure that we met at the beginning of the book expressing himself so frankly on the subject of love. But thanks to Mr. Darcy's transformation, and that of his bride-to-be, he's able to do just that. And not before time, we might think.