Fitzgerald was a Midwesterner; he was born in Minnesota and spent his youth there. In many of his novels and short stories, including The Great Gatsby, the West symbolizes certain values. Nick, a Midwesterner, is meant to represent a place where people are polite, considerate, hard-working, family-centered, and relatively socially conservative. Nick describes how his parents raised him to be grateful, fair-minded, and honest, and for the most part, his thoughts and actions align with those virtues.
The East, as represented in the novel, is a place of opportunity (Wall Street) but also the locus of many kinds of corruption. Here, Fitzgerald places criminality (Gatsby and Wolfsheim), infidelity (Tom, Daisy, Gatsby, and Myrtle), dishonesty (Jordan), and complete disregard for the social control attempted by Prohibition by all the drinkers at all the parties. Tom and Daisy, who choose to make the East their home, are wealthy, self-absorbed, and rotten parents—the opposite of the people who raised Nick and taught him how to conduct himself.
At the novel's end, Nick retreats to the Midwest to recover from the decadence of his brief sojourn in the East.