Consider Nick’s role as narrator and witness. What draws him to these people in the spring and what makes him wish to leave Long Island by the fall?

Nick Carraway is attracted to the glamorous lifestyle of New York City. At one point, he observes the “inexhaustible variety of life.” This line suggests that Nick delights in the diversity of experience, personality, and even location that a cosmopolitan life offers him. Nick is enchanted with the infinite possibility he sees amid the bustling city. He embarks on a career on Wall Street, likely because he thinks it is a lucrative, exciting venture that contrasts sharply with his monotonous life in the Midwest.
He only really interacts with the Buchanans since Daisy is his distant cousin and Tom is a former peer at Yale. Although Nick's narration focuses largely on his interactions with the Buchanans, Jordan Baker, and Gatsby himself, Nick does mention at several points that a significant amount of time passes between some of the encounters. Even so, he does choose to continue hanging around with them, even after discovering unsavory things of which he does not approve: Tom’s mistress, Jordan’s lying, Gatsby’s shady business dealings. This suggests that he is wrapped up in their saga, perhaps even entertained with it all.
However, Nick grows disenchanted after Gatsby’s murder. He states that the “east was haunted” for him. He realizes the insatiable greed for control that Tom exhibits, as evidenced by his asserting ownership of Daisy—although he still has multiple affairs during their marriage. He detests the judgmental attitude with which society regards Gatsby: everyone enjoys his lavish parties, yet no one cares to attend his funeral.
Nick famously states that Daisy and Tom are “careless people” who “smashed up things and creatures and then retreated back into their money or their vast carelessness.” This quote is emblematic of Nick’s disillusionment; while he once viewed the wealthy New York scene as glamorous, he now sees it for what it truly is. Nick believes it offers nothing but unfulfilling, uncaring narcissism.