How do we know right away that Nick Carraway is a reflective person?

Nick is a pretty credible character/narrator. He doesn't have any eccentricities and seems grounded. He was educated at the Ivy School Yale and fought in WW1. Nick isn't a character who needs anything. He seems to have enough money to settle down in West Egg. He has come to New York for work and to learn the bond business; he doesn't seem to be in a hurry to get starte. For much of the novel Nick is more or less a flat character. We can see life in West Egg through him. Nick observes situations without giving judgment, at least in the beginning parts of the novel.  This allows the reader to trust Nick as a narrator and finally a character in the novel.