The desire for the American Dream represented the demise of America, where hard work and good ethics were abandoned for wealth and the good life. The lesson could not be clearer; namely it is indispensable for the narrator to bring the 'outside reality' into focus.
Ultimately, Nick's major character trait — reserving judgment — allows him to be almost an "invisible" narrator, similar to a traditional third-person omniscient point of view.
After Gatsby's death there remains nothing in the East but void and emptiness: the only music and laughter that Nick can hear are imaginary, hallucinatory: 'I spent my Saturday nights in New York because those gleaming, dazzling parties of his were with me so vividly that I could still hear the music and the laughter' p First Person Peripheral Narrator Nick Carraway is our first-person narrator, but he's not the center of the story—and that makes him a peripheral narrator, someone who's always on the outside looking in.
On the opposite, Nick goes through different stages as lie tells the story.
He is spineless not very brave and easily influenced. When Gatsby dashes into the kitchen, Nick is made privy of his companion's feelings.