At the beginning of chapter 1, Nick quotes his father, who told him, "'Whenever you feel like criticizing any one, [...] just remember that all the people in this world haven't had the advantages that you've had.'" However, despite the fact that Nick thinks he's "inclined to reserve all judgments," he actually spends the majority of the book passing pretty casual judgment on everyone he meets. For example, as he leaves Tom and Daisy's house, he "was confused and a little disgusted as [he] drove away," having learned what kind of snobbish and self-centered people they really are.
The more often he comes in contact with Tom, Daisy, and Jordan Baker, the less he seems to be affected by their complete lack of personal integrity, their racism, Tom's infidelity and willingness to strike a woman, and so on. When he arrives to New York with Tom and Myrtle, Tom's mistress, they disappear into the bedroom to have sex, leaving him alone on the couch. He admits, "I have been drunk just twice in my life, and the second time was that afternoon [...]." Perhaps his need to get drunk hints at his early discomfort with their attitudes and behaviors. But even then, he says he "was within and without, simultaneously enchanted and repelled by the inexhaustible variety of life."
As the book continues, Nick seems to become more and more inured to what was, at first, shocking and disgusting to him. Despite helping Tom to cover up his affair and despite his awareness of Jordan's complete lack of personal or professional integrity and his growing attraction to her, Nick still feels at the end of chapter 3 that he is "one of the few honest people that [he] has ever known." It seems more likely that this was a quality he possessed before he moved to West Egg.
Despite this perception of himself, he is also willing to help Gatsby to lure Daisy to Nick's house in West Egg so that Gatsby will have an opportunity to reconnect with her. Once she arrives, he is happy to leave them alone, despite the fact that her presence there surely qualifies as a deception of Tom and leads to many acts of infidelity on her part.
In short, the fact that Nick becomes and more and more willing to immerse himself in this world where deception and arrogance are de rigueur shows us just how much he's changed. He's no longer the honest, non-judgmental person he believes himself to be or once was.