how has nick matured by the end of the novel?

Although Nick comes from a privileged background, he proves to be a reliable narrator. He sees the filthy rich as well as the working class, more or less, objectively. Nick initially is enchanted to be one of the "beautiful people." He "succumbs to the lavish recklessness of his neighbours". By the end of the novel,however, Nick has had enough of the superficial vapid people that call themselves the upper crust. Nick sees himself as a realist who was able to catch himself before becoming what he loathes.