An axiom of fiction writing is "write what you know," and Fitzgerald's outsider heroes or narrators reflect the author's own social location. Although he made it into the prestigious Princeton University, Fitzgerald's Minnesota roots were neither wealthy nor particularly well connected, and he engaged in self-destructive behaviors, such as flunking out of Princeton when he seemed en route to a successful career there. He also felt potentially inadequate and peripheral when Zelda's parents would not approve his marrying their daughter until he made a success of himself. Although he did, writing the best-selling This Side of Paradise and becoming the center of a bohemian 1920s world in France, he maintained the feelings of a person on the periphery.
This is probably best expressed in the characters Nick Carraway and Jay Gatsby in The Great Gatsby. Nick, like Fitzgerald, is the contemplative voyeur, watching and recording the dramas of other people's lives. Nick feels both peripheral to the charmed circle of vast wealth in which Tom and Daisy live, and peripheral to the grand drama of Gatsby's love of Daisy. Nick, the would-be stocks and bonds broker, wanders the streets of New York City in lonely exile, gazing in at other lives. This both alienates him and yet supremely positions him to be a commentator on his world.
Gatsby reflects the outsider, Fitzgerald, wanting to be let in—into the life of the young woman who is out of reach, into the world of wealth and education that Nick floats so easily through.
Fitzgerald was able to marshal his sense of being an outsider into a series of brilliant novels, catching a zeitgeist many of his generation felt and many today still do.