During the war years, Daisy had waited for Gatsby. But after two years, she started seeing other men. She marries a rich man and begins a life, first in Chicago and then in New York.
Gatsby follows these developments, and he plots and plans and dreams and remains convinced that Daisy still loves him. Nothing in his life — not his wealth, his showy estate, his flashy car, nor even his status among men — has any value to him other than the value that it may have in Daisy’s eyes.
He is sure that she will leave her husband, and come back to him to restart the exceptional romance that only the war interrupted.
We learn all of this — both in the book and movie — through the eyes of Nick Carraway, Daisy’s cousin and Gatsby’s friend. Nick is smart and perceptive, and he worries about people’s lives and what people can do to themselves and others.
He wonders if Gatsby is going to ask too much of Daisy, and if she and her philandering husband are happier and more content with their gaudy, meretricious lifestyle than either we or Gatsby thinks.
He warns Gatsby that “you can’t repeat the past,” to which Gatsby replies with incredulity and certainty, “Why of course you can.” Gatsby’s inability to live in the present began when he separated from Daisy, and his emotional capacities became compromised and handicapped at that point. After Daisy, the direction and coherence of his life depended upon his conviction that one day he would be reunited with her.
There is another character in the book, a woman named Jordan Baker, who has a different reaction to events in the past. Traumatized, disappointed, or hurt too often in her own life, she lives determinedly only in the present. Toward the past, and even toward the future, she is cynical, contemptuous, and without sentimentality or expectation.