Nick is relaying his father's advice " Whenever you feel like criticizing anyone, just remember that all the people in this world haven't had the advantages you've had."
The main purpose of Nick's character is to function as an objective viewpoint of the lifestyle at East Egg. For much of the story he does give us a middle class objective view of the excess of the filthy rich. Nick's perspective gets a little blurred as the story ends but he still works well as our "guide" in the story.
http://www.gradesaver.com/the-great-gatsby/q-and-a/does-nick-reserves-judgement-104047/
