For most of the book, Elizabeth does not like Mr. Darcy at all, but this changes as she begins to learn more about him.
When she first sees Mr. Darcy, Elizabeth thinks he looks very severe and unfriendly, and once he insults her when talking to Mr. Bingley, she is offended.
She is tolerable, but not handsome enough to tempt me; and I am in no humor at present to give consequence to young ladies who are slighted by other men. You had better return to your partner and enjoy her smiles, for you are wasting your time with me.
As the story goes on, she perceives all his actions toward her as trying to make fun of her or upset her. He stares at her constantly and she thinks he is looking for fault in her. He sits near her and she wonders why he can't just leave her alone.
When Mr. Darcy snubs Mr. Wickham, Elizabeth thinks that Mr. Darcy must not like Mr. Wickham because he disapproves of him just as much as he disapproves of her.
This all changes after he proposes to her.
At first, she tries to understand his behavior and what would make him propose to her when she was so sure he disliked her as much as she disliked him.
The letter he gives to her explains all the issues she brought up when she rejected his proposal. He tells her about what happened between Wickham and his sister and he owns up to being involved in taking Bingley away from Hertfordshire.
As she starts to reexamine his behavior, she becomes unsure of what kind of man he could be. But once she meets him again in Derbyshire at Pemberley, her opinion slowly starts to change. She sees how he is with his sister and how at ease he is when he is in his own environment.
The ultimate test comes when she comes to find out that he helped save Lydia from ruin by making sure Mr. Wickham married her. She also suspects that he had a role in why Mr. Bingley comes back to Hertfordshire and proposes to her older sister, Jane, after Darcy's advice had torn them apart before.
And by the time he proposes to her a second time, Elizabeth realizes her feelings for him have completely changed and acknowledges that she now loves him.