In The Great Gatsby, the east and the West (the Midwest in particular) become cultural opposites for Nick Carraway. He is drawn to the East Coast because it seems exciting, lucrative, and modern.
All of the excitement of the East Coast evaporates once Nick witnesses Gatsby's tragedy and how the search for money and prestige only leads to loneliness and death. He notes there is a "quality of distortion" to life in the East in general. Amorality and pleasure-seeking are prevalent. The divide between the rich and the poor is dramatic, with the wealthy living well and the poor living hand-to-mouth in the aptly titled "Valley of Ashes." People become playthings, a sad fact apparent in Tom's relationship with his mistress, Myrtle, and even in Daisy's reignited affair with Gatsby.
For Nick, the Midwest comes to represent honesty, innocence, and old-fashioned values, everything he believes the East Coast has rejected in favor of modern decadence. Take note of some of the details of his reminiscing about the Midwest in chapter 9: he mentions,
I am part of that [Middle West] ... a little complacent from growing up in the Carraway house in a city where dwellings are still called through decades by a family’s name.
Nick finds a sense of continuity and loyalty in the Midwest that eludes life on the East Coast entirely, where people will attend a man's parties en-masse but avoid his funeral when he dies.
He feels that both he and the other major characters from the West (Gatsby, Daisy, Jordan, and Tom) are ill-suited to the heartless and fast-paced East Coast world, even if they seem to have been content or tried to adapt themselves to its values. Daisy is unhappy with her life, trying to mimic the thoughtless giddiness expected of a modern woman living it up in the city. Tom's life is meaningless in the wake of his successful career as a college athlete and he tries to fill the void with affairs. Jordan's aloofness is a mask that protects her from being hurt. And Gatsby's earnest, romantic belief in the American Dream does not gel with the harsh, unfeeling world of East Coast life.
After his disillusionment in the east, Nick decides to return to the Midwest.