Nick's description of himself in the beginning focuses on the change that has occurred because of the events in New York. He explains that he comes from a well-known working family, well-off but not extremely wealthy, and that before going to New York, he was a tolerant person to whom people told their secrets.
...after boasting this way of my tolerance, I come to the admission that it has a limit... When I came back from the East last autumn I felt that I wanted the world to be in uniform and at a sort of moral attention forever; I wanted no more riotous excursions with privileged glimpses into the human heart.
(Fitzgerald, The Great Gatsby, mrbye.com)
The events he experienced changed him; he doesn't want to see people's secrets or attend parties, but to live in peace and quiet. He has seen the underbelly of Old Money, with their prejudices and derision for people who work for their money -- as he himself does -- and he is no longer the idealistic young man he was before. He also learns that he has no romantic notions about people and life; he refuses to see Jordan Baker again because he knows that she is not committed to an honest relationship. Essentially, his description is of a young man, just entering mental adulthood, whose innocence has been mostly shattered by the selfish people he has met.