Lydia is the rebel of the Bennet girls. She is headstrong and persistently ignoring advice, "Vain, ignorant, idle, and absolutely uncontrolled," as Elizabeth puts it (ch. 41). The disrespect towards her family and herself reaches a peak when she runs off with Wickham whom Elizabeth has learned is a man of poor character (despite his military background). Elizabeth fears that Lydia's actions "will be fixed, and she will, at sixteen, be the most determined flirt that ever made herself and her family ridiculous" (ch. 41). This is an issue that each woman faces in Pride and Prejudice, maintaining the image of their family.
While no Bennet does the potential damage that Lydia has, the girls are pressured, mainly by their mother, to get married or forever be a burden on Mr. and Mrs. Bennet. Even Charlotte Lucas must marry to protect her image and the image of the Lucas name as a whole. This pressure is what forces Lydia and Wickham to marry despite the fact that their fling was purely of passion. Wickham luckily is offered a monetary push in his decision to marry Lyida, which due to his debt, seals the deal. And although Lydia has not made "her family ridiculous," she has confined herself permanently to a man she desired for the moment.
Elizabeth internally processes their union after the two are found: "How Wickham and Lydia were to be supported in tolerable independence, she could not imagine. But how little of permanent happiness could belong to a couple who were only brought together because their passions were stronger than their virtue, she could easily conjecture" (ch. 50). This second marriage is the least respectable in Elizabeth's eyes since it is not based on character but desire which is not permanent.
Through highly valuing character, Elizabeth seeks to find happiness. In her discussion with Lady Catherine in Chapter 56, Elizabeth says she is "only resolved to act in that manner, which will, in [her] own opinion, constitute [her] happiness" (ch. 56). This definition of action is well represented in the process that lands her in the arms of Mr. Darcy.
Mr. Darcy appeared distasteful at first, even to the reader, through his remark that Elizabeth was "not handsome enough to tempt" him (ch. 3). Further, his case was not helped when the "truth" arose about his interaction with Wickham and his advice to Bingley not to continue to pursue Jane. When he proposes to Elizabeth in chapter 34, both the reader and Elizabeth herself are surprised to hear Darcy profess his love for her: "In vain have I struggled. It will not do. My feelings will not be repressed. You must allow me to tell you how ardently I admire and love you" (ch. 34). This is obviously either a change of Darcy's mind or a breaking of pride in admitting to love someone of a status that would "ruin him in the opinion of all his friends, and make him the contempt of the world" (ch. 56) as Lady Catherine put it in the continuation of her discussion with Elizabeth in chapter 56.
It is Elizabeth's mind that now must change, as she declines his proposal in chapter 34. As she begins to learn the reality of Darcy's actions with Wickham as innocent and forgives him for breaking ties with Jane and Bingley (since she knows Jane will also forgive Bingley), she is allowed to reconsider Darcy's character. This pivotal change is made at his estate when she is "delighted. She had never seen a place where nature had done more, or where natural beauty had been so little counteracted by an awkward taste. They were all of them warm in her admiration; and at that moment she felt that to be mistress of Pemberley might be something" (ch. 43)! Despite the fact that her initial reconsideration is spurred by the material attributes of Darcy, it's the material desire that allows his character to break through her prejudice. The respect those closely related to Darcy, his love for his sister, and his (now) agreeable countenance, were the factors that Elizabeth so craved.
Elizabeth's desire was not of temporary lust as Lydia or of security as Charlotte, but for happiness. The greatest message that Austen provides through the trials of the women in Pride and Prejudice is to follow your heart. Adhering to social responsibilities or bodily passions will only get you so far. Contentment is not the goal, nor is assuaging thirsty desire - but permanent happiness, that is the aim.