"Gatsby believed in the green light, the orgastic future that year by year recedes before us. It eluded us then, but that's not matter- tomorrow we will run faster, stretch out our arms farther . . . And one fine morning--" Why does Fitzgerald leave this sentence unfinished?
The entire novel is the story of Gatsby pursuing his dreams. The green light, a symbol of life and hope that tomorrow will be better, was always just out of Gatsby's reach. In reality that hope, that dream was unachievable, "already behind him, somewhere back in that vast obscurity beyond the city." But Gatsby never lost hope and never stopped dreaming of the day when he would achieve it.
Fitzgerald left the sentence unfinished because he wanted the reader to imagine the continuing chase after that dream. As the reader, as Nick, as Gatsby "run faster, stretch out our arms farther..." there is always hope - hope that we might actually catch whatever it is that we're dreaming of. Even when it's useless because we are struggling against the factual world, "boats against the current, borne back ceaselessly into the past," Fitzgerald wanted to leave his readers with that unceasing optimism that shaped Gatsby's existence.