Most of the characters in Pride and Prejudice have a perspective of what a person's ideal character, meaning main traits of a person's nature, should be like. This is especially true of our hero and heroine, Elizabeth and Darcy. Both know and understand that there are ideal character traits and not so ideal character traits, and both feel ashamed when they learn that they have not been acting in line with their own ideals. Hence, both Elizabeth's and Darcy's ideals shape their decisions to change their behavior.
For Elizabeth, part of what makes up an ideal character is the ability to accurately judge others. We see Elizabeth criticizing her sister Jane for Jane's inability to see anyone's faults. As Elizabeth phrases it, "All the world are good and agreeable in your eyes. I never heard you speak ill of a human being in my life," and Jane concedes that she "would wish not to be hasty in censuring any one" (Ch. 4). However, Elizabeth is right in thinking that the disadvantage of Jane's perspective is that Jane is incapable of judging people's actions correctly, which can lead a person to make bad decisions, such as deciding not to expose Wickham in case he is trying to re-establish his character. Hence, Elizabeth values her own abilities to judge and discern, thinking that they are far better than her sister's. Yet, Elizabeth soon discovers that her abilities to discern are really not what she thought they were, despite the fact that she considers the ability to discern an ideal character trait. In reality, she misjudges Wickham to be the best man she's met simply because he is friendly and conversational and misjudges Darcy to be a despicable person simply because he is reserved and was spoken ill of by Wickham. Elizabeth, seeing how she has fallen short of her ideal, soon amends both her opinions and her actions.
Darcy also believes in ideal character traits. He values having pride in one's ethical conduct and "superiority of mind" (Ch. 11). As he tells Elizabeth later on in the book, he also values good principles. One of the principles he holds is that he must always speak the absolute truth, which is one reason why Darcy talks about how inferior he believes Elizabeth and her family are to himself and how marrying her would be a "degradation" of his pride during his first proposal. Also, since he believes he has upheld his principles, he is absolutely shocked when Elizabeth calls him arrogant, conceited, selfish, and accuses him of not behaving in a "gentleman-like manner" (Ch. 34). In addition, he believes that his ideal character trait is to uphold his principles; therefore, he is absolutely ashamed and mortified by Elizabeth's chastisement and, later, by the things he said to her during his proposal. Later, Darcy makes an effort to act with less pride and less conceit. Therefore, even Darcy's ideals make him change his behavior.