In a lot of ways, Fitzgerald uses "Winter Dreams" to comment on the American myth of the self-made man. He's basically saying, yes, it is possible for an American to come from nowhere and make tons of money; but don't expect perfection. This self-made man may find it disappointing (and soul-destroying) once he's reached his goals.
But here's the thing: Fitzgerald also doesn't present the United States as a giant, unified block. There is some major regional difference in this story. The East Coast represents old money and high finance, while the Midwest (or "Middle West") represents energy and ambition. Do you think Fitzgerald is making a judgment call here? Is one better than the other?
Dexter comes from Minnesota, but in order to feel like he is really gaining social status, he has to go to college at an elite institution back East. Even though Dexter makes it big as a businessman in the cities of Minnesota, he has not really proven himself financially until he can take his cash to Wall Street, in New York. But Dexter's origins outside of the East Coast are also what make him "the rough, strong stuff from which [the upper class in the United States] eternally sprung" (3.1).
Once Dexter gets to Wall Street, his business associate Devlin comments, "So you're from the Middle West […] That's funny – I thought men like you were probably born and raised on Wall Street" (6.3). Now that Dexter has succeeded in New York, it's like his origins in the Midwest (where he dreamed his winter dreams) have faded away. So Fitzgerald is drawing a comparison between Dexter's boyhood dreams and Minnesota. With his success, Dexter has, in a sense, lost both of them. The closer he has grown to that American center of finance, Wall Street, the further away he has traveled from the ideals of the boy he once was. Wait, we thought Los Angeles was the city of lost dreams...