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.
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