His disillusionment is with those that seem to embody the fulfillment of the American Dream and who feel no responsibility to their fellow man. Those, like the Buchanans, who are "careless people" who
smashed up things and creatures and then retreated back into their money or their vast carelessness or whatever it was that kept them together, and let other people clean up the mess they had made . . .
are the ones who earned Nick's contempt. At the end of the novel, Nick looks across the bay and tries to see it as those first settlers saw New York. It was a land that
pandered in whispers to the last and greatest of all human dreams.
But the realization of those dreams in the form of the Buchanans results in empty and careless lives. Daisy wonders aloud what do people plan, and Tom attempts to find meaning in polo horses and mistresses. The result of the work of the early settlers is excess, stagnation, boredom, and selfishness.
Nick is not disillusioned with Gatsby because Gatsby was still reaching for something beyond him--no matter how unattainable.
Gatsby believed in the green light, the orgastic future that year by year recedes before us.
Gatsby had hope that the future would be better. Like Mrytle, Gatsby was striving for more than the status quo. The others remained stagnant, entombed in their vast wealth.
Nick states early in the novel that it is not Gatsby, but "what preyed on Gatsby, what foul dust floated in the wake of his dreams," that disillusioned him. As Nick will go on to show us as his tale unfolds, it is the Buchanans, primarily Tom, but Daisy too, and the way their vast wealth insulates them from any consequences of the lives they wreck, that disillusions him. He is stunned and angered by how carelessly a life like Gatsby's can be thrown away and forgotten.
More than just the Buchanans sours Nick. His disillusionment includes Jordan, from whom he "turned away" at their final meeting, as well as all the people who came to Gatsby's lavish parties "by the hundreds" but didn't bother to show up for his funeral. Nick's disillusionment expands to encompass the entire East Coast, which becomes distorted for him like a "night scene" in an El Greco painting. He pictures a drunken woman carried on a stretcher when he thinks of the East, and in this tableau, "no one knows the woman's name and no one cares." As he puts it, "the East was haunted for me like that." Nick retreats into a childlike storybook "Middle West" of "street lamps and sleigh bells in the frosty dark."
However, Nick's story also offers hope: he says that the "foul dust" only "temporarily" ended his interest in other people. In writing this story, he clearly hopes to capture and memorialize some of what made Gatsby great in his eyes. Rather than being a source of disillusionment, Gatsby is the only person who remains romantically "pure" in this saga, because of the way he holds onto his dream.