What was F. Scott Fitzgerald's main purpose for writing The Great Gatsby?

Despite its slim size, The Great Gatsby encompasses a diverse array of important themes, commenting on everything from the state of capitalism to gender relations. As such, it's very difficult to pinpoint a major purpose. That said, if there is a major purpose in the novel, it is most likely Fitzgerald's critique of the classical American Dream.

Simply put, the American Dream is the widespread notion that any American citizen can achieve happiness and fulfillment by simply working as hard as possible. Fitzgerald deconstructs this idea by showing that, though Gatsby works hard and acquires a vast store of riches, he does not ever achieve true happiness or fulfillment. For Gatsby, true happiness involves earning the lasting love of Daisy. However, though Daisy loves him in her own way, she is not able to love him as fully as he would prefer, and in the end Daisy abandons Gatsby. Thus, no matter how hard Gatsby works to gain material wealth, he ultimately dies alone, and so much of the novel's major purpose is to ultimately critique the mythology of the American Dream.