In "The Great Gatsby", what is the moral of the story?
The novel is really too complex to be reduced to a single "moral of the story" analysis, but one compelling theme seems to be Fitzgerald's assertion that romantic illusion cannot survive when pitted against reality. Gatsby's romantic illusions led him away from North Dakota in search of a new identity. He believed he could remake himself in an image of his own creation and that through the force of money, he could become someone else. Thus, Jimmy Gatz becomes Jay Gatsby.
When Gatsby meets, loves, and loses Daisy, she becomes his dream and the focus of his illusions. He believes that with enough money, he can become a part of her world, win her back, wipe out five years of time, and repeat their past. When Gatsby's illusions collide with reality, however, everything falls apart, and Gatsby is murdered. In the novel's conclusion, Nick meditates about Gatsby's conflict between illusion and reality:
He had come a long way to this blue lawn and his dream must have seemed so close that he could hardly fail to grasp it. He did not know that it was already behind him, somewhere back in that vast obscurity beyond the city, where the dark fields of the republic rolled on under the night.
The power of Gatsby's illusion is so strong, however, he dies still believing in it, waiting for Daisy's call.
I am going to assume that moral is meant instead of theme. A theme is a central idea or topic of a story, and a moral is the lesson that a particular reader winds up with after the story. Different readers might see a different moral of The Great Gatsby. It's possible that certain readers see the moral of this story as a straightforward "don't cheat on your significant other"; however, I think that is a shortsighted moral when thinking about the entire story and several of its themes. I think a better lesson/moral that readers should pull from this story is that the "American Dream" is an unrealistic and unattainable illusion. For many people (Gatsby included), the American Dream is a beautiful house with a yard, a beautiful spouse, and financial security. Gatsby has most of that. He doesn't have Daisy, but his pursuit of that one additional "thing" to have in his life is what ultimately destroys him. Gatsby isn't happy with what he has, because it doesn't fit his preconceived notion and dream. I think a moral of the story is that kind of thinking can be dangerous.
He had come a long way to this blue lawn and his dream must have seemed so close that he could hardly fail to grasp it. He did not know that it was already behind him . . .