In Chapter I of the novel, Nick introduces Gatsby, defining him in terms of his most significant trait: Gatsby is a romantic. From his earliest years in North Dakota, he had longed for a life filled with beauty, glamor, and a heightened sense of possibility. He lived for his romantic dreams, and his dreams defied the realities of his birth.
When he meets Daisy in Louisville, he does not intend to fall in love with her, but slowly he is drawn into her exalted world of wealth and the glamor that surrounds those who own it. She is beautiful and desirable; many young officers vie for Daisy's company. The fact that other men want Daisy so much makes her all the more desirable for Gatsby. When he gives himself up to loving her, she becomes the incarnation of all his romantic dreams. She is all beauty, mystery, and enchantment for him.
Chapter VI and Chapter VIII include passages of exposition that deal very specifically with Gatsby's early relationship with Daisy and the circumstances under which he fell in love with her.
As a young man, Gatsby (then James Gatz) was poor and extremely ambitious. Dan Cody took him on board his yacht and taught him how to act rich, but then left him penniless... so he is still obsessed with money and social class. Daisy, to him, represents everything he never thought he could have - the huge white house, the white dress (symbolizing purity - "she was the first nice girl he had ever known"), the social status. He believes he is in love with her but really he is most drawn to her life and all of the things he never had and always wanted.
Gastby was obsessed with money ever since he was left with nothing from Dan Cody, because Daisy had money, he was instantly drawn to her. She represents everything that he thought he would never have - a rich life sytle.