That's Gatsby's biggest character flaw. He's a romantic. He thinks by becoming everything Daisy every dreamed about (wealthy and power and affluence) that he can just come in and sweep her off her feet. he's not prepared for the reality of her situation and of her conflicted feelings. He doesn't understand them because he's romanticized her as a heroine, when in fact, she's just a silly girl who has made some rather bad decisions in her life.
