We learn a lot about our narrator, Nick, in the first few pages of the novel, don't we? In short, we learn that at one point, he was young and vulnerable. (In many ways, he is STILL quite vulnerable and able to be taken advantage of, especially by Gatsby himself.) Let's look at some quotes from the very beginning:
In my younger and more vulnerable years my father gave me some advice that I’ve been turning over in my mind ever since.
“Whenever you feel like criticizing any one,” he told me, “just remember that all the people in this world haven’t had the advantages that you’ve had.”
This also tells us that Nick wasn't born poor. He was born in the Midwest into a good family and with the prospect of a good education. Therefore, here he also describes himself as having a great respect for his father. Still, Nick was vulnerable and as such, he says, "made me the victim of not a few veteran bores." Yet, as a young man who has respect for his father, Nick was "the only honest person" he knew.
It is also at the beginning of the novel that Nick describes himself as being completely bored of the honest Midwest and looking for more excitement in life, so he decides to move to New York to become a "bond man." Nick, then, as an honest person, is not above looking for exciting risks. Moving to New York, and moving next to Gatsby, was one of them.
Nick is from a well-to-do family but lives rather modestly. He describes himself as "the only honest person" he knows. He is the moral center of the book, although he tends to be corrupted a bit by his neighbors' and Daisy's reckless and extravagant ways as the book progresses. He is disgusted by the "amusement park" behavior of Gatsby's guests, yet he is attracted to the lifestyle and Jordan Baker--both which will cost him his innocence and modesty later on in the book.
Nick begins his story by establishing the fact that he does not pass judgment on people--a quality instilled in him by his father. His father's advice to Nick during his "younger and more vulnerable years" is to "remember . . . the advantages you've had" when he "feel[s] like criticizing anyone."
Because of his father's values, Nick is "inclined to reserve all judgments" on others, which makes him attractive to people with stories to tell. As a result, he often finds himself "privy to the secret griefs of wild, unknown men"--people like Jay Gatsby.