the universe" and that it was for this reason that he "decided to go East and learn the bond business."

Really, then, Nick moves to New York because, having been changed by his experiences during the war, he cannot adjust back to normal everyday life. He is restless, perhaps even bored, and desperately needs a change. Still, Nick is level-headed and practical, and he does not go to New York quickly or impulsively; rather, he travels east only after discussing the matter with his entire family, agreeing with his father about financing terms, and other "various delays." We also see his practicality in his choice of "the bond business" as employment. Nick says, "Everyone I knew was in the bond business, so I supposed it could support one more single man." Hence, he chose this particular business opportunity not because it was necessarily interesting or exciting, but because it was a means to an end—that he might start an entirely new and different life in New York.

As you say, you can find this in Chapter 1.  In my edition of the book it is on page 7, in the part where he is telling about his family background.  Nick says that he has come to New York (he says to the east) in order to learn the bond trade.  What this means is that he will, more or less, become a stock broker (stocks and bonds are not the same thing, but both are types of investments that can be bought and sold).  Nick figures that he can make it selling bonds because everyone else is doing it so why not him?

By the end of the novel, Nick's experiences in New York have disillusioned him. The people he has encountered there, while sophisticated and lively, are also superficial and amoral. The rich shirk responsibility and abuse the poor. Gatsby, flawed as he was, was abandoned in the end by the woman he loved and ignored by his enchanted party guests the moment he died. Friendship and love mean little in a world where money and power are the bottom line, and Nick finds he's no use for its "quality of distortion."

Nick goes into detail as to why he now prefers the Midwest to the East. He associates the Midwest with a sense of innocence, particularly his childhood:

That's my Middle West—not the wheat or the prairies or the lost Swede towns, but the thrilling returning trains of my youth, and the street lamps and sleigh bells of the frosty dark and the shadows of holly wreaths thrown by lighted windows on the snow. I am part of that, a little solemn with the feel of those long winters, 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.

For Nick, this honesty and continuity are preferable to the glitz and frenzy of the East Coast. He even goes as far as to argue Gatsby, Jordan, Daisy, and Tom were also ill-suited for the East Coast because they, like Nick himself, all craved something more than just going to the next party or what was the latest in fashion. Even Tom, with his genuine mourning over Myrtle's death, is shown to have some care for another person, a quality which is otherwise missing from the East Coast's social milieu.

Nick Carraway experiences the fast-paced, materialistic, immoral lifestyle of the wealthy citizens living in New York City and Long Island and cannot wait to travel back home to the Midwest by the end of the novel. After befriending Jay Gatsby, experiencing his ostentatious parties, and interacting with Tom, Daisy, and Jordan Baker, Nick Carraway has had enough of the East Coast. Following Gatsby's death, Nick Carraway makes funeral arrangements and entertains the only guest at Gatsby's funeral, which is, sadly, Jay Gatsby's father. As a relatively moral man, Nick is disgusted with the careless, debased nature of his social group and the "foul dust" that preyed on Gatsby in the East. Before Nick tells Gatsby's story in hindsight, Nick gives the reader insight into his decision to return home to the Midwest by saying,

I came back from the East last autumn I felt that I wanted the world to be in uniform and at a sort of moral attention forever; I wanted no more riotous excursions with privileged glimpses into the human heart (Fitzgerald, 3).

Essentially, Nick feels that life in the Midwest is more pure and predictable than the materialistic, superficial, immoral East Coast, where Gatsby becomes a victim of selfish, greedy individuals and discovers that the American Dream is futile.

At the end of The Great Gatsby, Nick returns to the Midwest because he is disillusioned with life in New York. Specifically, he has come to realize that many of the people there are empty and "careless." Jordan, for example, has become engaged to another man (without first breaking it off with Nick) and Tom and Daisy have no interest in Gatsby's death. In fact, Tom admits to playing a role in Gatsby's murder by telling Wilson that Gatsby was driving the car that hit Myrtle.

In addition, Nick is also disillusioned because he realizes, through Gatsby's short life, that the American Dream is a destructive force. While Gatsby made himself rich and famous, the only person he truly desired was Daisy, but she had no intention of ever leaving her husband. Gatsby's desperation to "recreate the past" was nothing more than a fantasy: he could not achieve his dream, and for Nick this provides the impetus to return home. 

At the end of The Great Gatsby (Fitzgerald), Nick is completely disheartened by his experiences in New York.  He says that after Gatsby dies, "the East was haunted for me..."(185). He believes that he is best-suited to the mid-west.  He muses over the happy times he has there, the family, the traditions, houses that remain in one family for generations, the stability, and even the bracing air of the long winters.  He says that Gatsby, Jordan, Daisy, and Tom, too, were ill-suited for life in the east, possessing "some deficiency in common which made us subtly unadaptable to Eastern life" (184). They were all unfit for the frenzies of the New York Jazz age, the frenetic pace, the lack of loyalties, the disregard for tradition, the anonymity. 

It is important, I think, to be aware that F. Scott Fitzgerald was born in St. Paul, Minnesota.  He lived in many places throughout his life, New York, Delaware, California, and France, to name a just a few.  But reading The Great Gatsby, one cannot help but think that Nick Carraway is expressing Fitzgerald's longing for the mid-west he himself had begun in, a mid-west that remained just a dream, since he died in Hollywood, California, writing screenplays.  Fitzgerald is kind of literary equivalent to Hoagy Carmichael, whose songs have the same plaintive longing for the same part of America.  There is good reason to call the mid-west the heartland of the country.