The central conflict in the novel concerns Gatsby's dream of winning Daisy back and repeating their past as if they had never been separated. Gatsby wants to wipe out the previous five years, an impossible dream. Nick tries to explain to Gatsby that no one can ever repeat the past, but Gatsby refuses to believe it:
"Can't repeat the past?" he cried incredulously. "Why of course you can . . . I'm going to fix everything just the way it was before," he said, nodding determinedly. "She'll see."
As Gatsby continues to talk, Nick begins to understand the importance to Gatsby of his dream:
He talked a lot about the past and I gathered that he wanted to recover something, some idea of himself perhaps, that had gone into loving Daisy. His life had been confused and disordered since then, but if he could once return to a certain starting place and go over it all slowly, he could find out what that thing was . . . .
The conflict is resolved when Daisy refuses to tell Tom Buchanan, her husband, that she never loved him, choosing instead to abandon Gatsby again and stay in her marriage. In the hours leading up to his death, Gatsby is still waiting for Daisy to call, refusing to recognize, acknowledge, or accept that his dream is not going to come true.
Gatsby has amassed a huge fortune in order to win the affections of the upper-class Daisy Buchanan, but his mysterious past stands in the way of his being accepted by her.