In The Great Gatsby, where does Nick, the narrator, come from and why?
Like Jay Gatsby and like F. Scott Fitzgerald himself, Nick Carraway comes from the Midwest (Minnesota). He describes his background in his introduction as part of Chapter 1.
"My family have been prominent, well-to-do people in this Middle Western city for three generations."
The significance of Nick's Midwestern background are two-fold. His origins make him an outsider to the social circles covered in the New York Times society pages, the glitzy and glamorous wealthy, eastern elite. Although Nick comes from an old family with some money, he is not from a family like Tom Buchanan's, which is true "old money."
Secondly, Nick's origin ties him to Jay Gatsby (and to F. Scott Fitzgerald as well). Gatsby goes east from the Midwest and Nick does too. In the end, neither of them succeeds in making a life there and neither of them seems to really belong. Notably, both Nick and Gatsby are reflections of Fitzgerald's biography and both seem to mirror Fitzgerald's reputed worries about "making good" and proving himself in such a way as to secure a permanent stature as one of the American elite.
Looking more directly at the text, we should not overlook the fact that Nick sympathizes with Gatsby in the end and, throughout the story, Nick is as much of an outsider/visitor at Gatsby's parties as Gatsby is. They each desire a woman and, perhaps, a place in the high society of the east, but they end up alone.
Is this connection explained entirely by the similar geographic backgrounds of these two characters? No, it isn't. But the fact that Gatsby and Nick Carraway each come from a more humble background than the likes Tom Buchanan serves to underscore the ultimate intimacy of these two characters - in terms of their respective stories and their positions as arrivistes on the scene, visitors to the party, as it were.
Narrator Nick Carraway came east from the Midwest to make his fortune working in the bond market. (He's a young man trying to establish a new career, and, though he doesn't say so directly, have a little adventure far from home.)