In The Great Gatsby why does Nick want people to think he's honest and not judgmental?

If Nick is an honest narrator, that makes his account more believable. To most Americans at the time The Great Gatsby was written, the opulent world of East and West Egg might as well have been on another planet. The kind of fantastic wealth so liberally on display in the story was the preserve of a tiny minority. At the same time, most people then as now had a certain fascination with the lifestyles of the rich and famous. To guide us through this glamorous, crazy world, we need someone more resolutely ordinary than the people he meets, those whose gilded lifestyles he seeks to portray.
Enter Nick Carraway, a man from a relatively humble Midwestern background. Nick has the advantage of being a part of neither the East Egg nor the West Egg crowd. At the same time, he moves effortlessly between them, giving us an insight into their morals—or lack of them—strange customs, and modes of behavior. Nick's honesty allows him to take on the role of a kind of anthropologist, giving us a highly accurate, disinterested account of his many astute observations.