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.