Gatsby's capacity for hope and for insistence on pursuing a difficult dream is the basis for Nick's ultimate respect for Gatsby.
Nick sees Gatsby as a man who is not dedicated to the truth, per se, or to the trappings and limitations of accepted reality, but instead is dedicated to shaping his own world so that he one shining dream can come true. Gatsby loves Daisy (or the idea of marrying her) so much that he is willing to believe that he can repeat the past.
He says exactly this to Nick.
Concerning his behavior with Daisy, Nick tells him he can't repeat the past. “Can't repeat the past,” Gatsby replies, “Why of course you can!”
He dedicates his life to this end (i.e., recapturing Daisy) and, effectively, his entire story is the story of this pursuit.
The qualities of character that allow Gatsby not only to say this to Nick but to truly mean it are the qualities that in the end separate Gatsby from the other characters in the world of the novel.
While Gatsby is outwardly caught up in the same desires for wealth, glitz and glamour as other people, he is inwardly untouched by the shallowness of this material orientation.
Gatsby's single-mindednes is wrapped up in what Nick sees as a kind of innocence. This is an innocence that crime (bootlegging, etc.) cannot touch. This is an innocence that slander cannot touch, even as Gatsby is attacked in the press and to his face, called a fraud, and derided. None of this can diminish Gatsby's belief in his dream and therefore none of it can sully Gatsby's essential innocence.
In Nick's commentary throughout the novel and especially at the end, we can see clearly how Gatsby's penchant for hope influences and impresses him. We also see Gatsby's passion for hope rub off on Daisy. She is nearly convinced to leave her husband and nearly convinced that, with Gatsby, she can reclaim years of her life and establish a relationship of true romance and love.
Though Daisy is more timid, or perhaps simply more practical, than Gatsby, we see her as sharing in some of Gatsby's hope: "like Gatsby, she carries the 'well-forgotten dreams from age to age.'”
Gatsby's idealism is stronger and more desperate than that of any other character. In its fatality, this idealism lends some nobilty and even some greatness to Gatsby's character, despite his flaws. For Nick, the nobility comes from the special innocence and willingness to pursue a singular vision that marks Gatsby's character.
Insofar as Gatsby represents the simplicity of heart Fitzgerald associated with the Midwest, he is really a great man.
In a final reckoning, Gatsby's hope inspires Nick to hold on to his sense of integrity and values, as we see in the action that follows Gatsby's death and funeral. For a brief moment, Gatsby's noble dreaming also inspired hope in Daisy.
However, for the other characters in the novel, Gatsby's "simplicity of heart" was something to be manipulated, to be derided, or a kind of mystery, an anachronism in a cultural era of (false) sophistication.