Nick is fascinated by Gatsby after their first encounter and is particularly impressed by his optimism and vigor. This first impression wavers but ultimately lasts throughout the course of the book.
While Gatsby is the titular character of F. Scott Fitzgerald's novel, he is not properly introduced until chapter three. By this point in the book, the reader has been inundated with gossip and idle speculation about Gatsby's early life and income.
When Nick finally meets Gatsby, he comments on his smile: "He had one of those rare smiles with a quality of eternal reassurance in it, that you may come across four or five times in life."
This distinguishes Gatsby from the old money individuals of Nick's acquaintance, who have none of Gatsby's sanguine outlook on life.