Elizabeth marries Darcy in Pride and Prejudice because she truly loves him. She has come to know him, she understands him better than she did when they first met, and she has developed strong feelings for him. Elizabeth is not the kind of young woman to marry for money and/or security, as her friend Charlotte Lucas does. We see this when Elizabeth turns down Mr. Collins's marriage proposal and rejects Darcy the first time.
Both men offer her a much more secure life than she can have as a single woman living in a family of women with an aging father. The family’s estate, Longbourn, is entailed to Mr. Collins, so he will inherit it upon the death of Mr. Bennet, and it would be within his right to turn Mrs. Bennet and her daughters out. Without a male relative to help them, the women will have an extremely difficult time supporting themselves. Yet Elizabeth is willing to risk the prospect of this future rather than marry Mr. Collins or Darcy because she does not love or respect either man at first.
Elizabeth first begins to change her attitude towards Darcy when she visits Pemberley. It is not just the estate that affects her view of Darcy. It is also his easy and loving relationship with his sister and the admiration of his housekeeper that makes her realize that she had jumped to some conclusions about him initially. Then, when Darcy saves Lydia and convinces Bingley to marry Jane, Elizabeth is convinced of his character and is in love with him.
Elizabeth Bennet is described as an opinionated, an intelligent, a playful, and a (more often than not) stubborn woman. When she first meets Mr. Darcy, she thinks that he is cold, arrogant, and egotistic. However, as the story progresses, we realize that her opinion might have been clouded with judgment and prejudice.
Mr. Darcy selflessly decides to help Elizabeth's younger sister Lydia when the family discovers that she practically ran away from home to marry the manipulative Wickham; Darcy brings her back and saves both her and her family's honor. He also encourages his best friend to pursue a relationship with Elizabeth's older sister, Jane, when he sees that their love is real.
His actions prove to Elizabeth that he is actually a kind, genuine, and considerate man who treats people with kindness and respect; most importantly, she realizes that she has fallen in love with him. Thus, when Mr. Darcy proposes to her for the second time, Elizabeth accepts.
By the end of Jane Austen's Pride and Prejudice, Elizabeth decides to marry Darcy because she has come to realize what a genuinely caring, selfless, goodhearted man Darcy truly is and that she had severely, prejudicedly misjudged him.
Elizabeth first begins to realize how severely she had misjudged Darcy after reading his letter to her. In his letter, he justifies his opinion that the Bennet family is beneath him by reminding her of all the ways the Bennet family members have acted with impropriety, especially Elizabeth's flirtatious younger sisters, her gossiping mother, and her father for failing to control his own family. More importantly, he corrects her judgement of him by giving his own account of why relations between himself and Wickham have grown cold. His own account is that Wickham refused to take the living the late Mr. Darcy left him, asked Darcy for £300 with the professed purpose of studying law instead, lived recklessly, and attempted to elope with Darcy's 15-year-old sister to try and gain her fortune. The information in Darcy's letter rocks Elizabeth to her core. She feels "absolutely ashamed of herself" for having so severely misjudged both Darcy and Wickham (Ch. 36). She is shocked to realize that, after having taken pride in her own "discernment," she could have been so "blind, partial, prejudiced, and absurd" (Ch. 36).
Elizabeth further realizes just how much she had severely misjudged Darcy when she visits the Pemberley estate while traveling with her Aunt and Uncle Gardiner. While being given a tour of the manor by the housekeeper, Elizabeth is amazed to hear Darcy being so highly praised by his servants. Yet, the most influential moment is when she learns from Lydia's gossip, as well as from her Aunt Gardiner's letter, that Darcy had bribed Wickham into marrying Lydia, thereby saving Lydia's reputation, as well as the reputation of the entire Bennet family. Elizabeth felt that Darcy's only motive for having done so was because he felt personally responsible for Lydia's situation since he could have publicly exposed the nature of Wickham's character but had failed to do so out of pride. Darcy's behavior was enough to tell her what a genuinely goodhearted person he truly is. Hence, by the time Elizabeth's father protests against Elizabeth accepting Darcy's proposal in Chapter 59, Elizabeth is able to reply, "I love him. Indeed he has no improper pride. He is perfectly amiable. You do not know what he really is," showing us just how greately her opinion of Darcy had changed throughout the novel (Ch. 59).