Why is it important for the reader to accept Nick's statement, "I am one of the few honest people that I have ever known."?
We need to be able to trust Nick because he is the narrator of the story. Although he potentially could be an unreliable narrator, he provides us with a description of the action and the characters' behavior that does not give us much reason to question his account. We are being introduced to a strange, exotic world here, one of opulent wealth and glamor. At times, it can seem almost like a fairy-tale world. In fact, the picture of the American Dream that The Great Gatsby presents is precisely that for many in the story. Indeed, most of the characters in the book are, to some extent, delusional. We need a clear-headed, truthful guide to take us on a tour of this crazy world, in which nothing is as it first appears. It is absolutely essential that if we are going to understand this world and the people who live in it, we need to be able to accept the truth of whatever it is that Nick is telling us. He is our way into the rarefied world of West Egg and East Egg; he is an outsider, just like most of the book's readers. If Nick can give us a truthful account of what he sees, then we can gain a much better understanding of what happens in the story, making us more deeply engaged with the characters and their situations.
The entire story of The Great Gatsby is told from the point of view of Nick Carraway. This means that all the actions, reactions, and consequences are described from his perspective. If the reader can not trust Nick's commentary or does not believe his version of the events that took place, then the reader is left with only a fictional account of the extravagant and wasteful lifestyles and relationships of a small group of self-absorbed individuals.
However, Nick goes to great lengths to establish that he is not a judgmental person, commenting that "Reserving judgments is a matter of infinite hope." Because Nick convinces the reader that he isn't judging the people he is telling about and because the reader accepts Nick's honesty in telling the story, the reader can be reassured that his/her impressions of the characters are valid. Based on Nick's truthfulness, readers can believe that the motivations and relationships portrayed in the book are real, and can identify with them in confidence that they are not being as foolish or self-deluding as the characters sometimes are.