Gatsby turned Daisy into his golden girl, a person who represented ideal perfection. He put her up on a pedestal and made her into something that she was never able to attain in reality.
When Jay Gatsby met Daisy prior to his leaving for the war, he realized that he had fallen hard for her. He never intended to fall in love with anyone, as that got in the way of his future endeavors. However, Daisy changed all of those for Gatsby. He left for the war and stayed in contact with her for several years. Daisy eventually married Tom Buchanan because she felt the need to have her life settled in that instant, and Gatsby was abroad.
Despite knowing that Daisy was married, Gatsby still built up his fortune, albeit illegally, in order to prepare a perfect place for Daisy to come live with him, leaving Tom in the dust. He spent the last five years building up Daisy so much into this idealistic woman that there was no way she actually lived up to that image. When Jay finally met Daisy in the middle of the book, he believed she was still his golden girl. Slowly, though, the facade that was Gatsby started to fade. Daisy realized that Gatsby was not who he always said he was. Due to Daisy moving out of Gatsby's reach, Gatsby soon began a rapid fall from grace and eventually died. Fitzgerald made it this way because Gatsby's ideal golden girl was no longer within his grasp.