He knew that Daisy was extraordinary, but he didn’t realize just how extraordinary a “nice” girl could be. She vanished into her rich house, into her rich, full life, leaving Gatsby — nothing. He felt married to her, that was all.
Chapter 6 mentions that Gatsby had “known women from a young age.” He’s dated (and slept with) people before, but it’s not until Gatsby dates Daisy that he feels hopelessly attached to somebody.
Nick suggests that it’s because Daisy is a “nice” girl—she’s from a proper and respectable family, and Gatsby feels fuller and richer for being part of the relationship. Without it, he’s “nothing.”
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