There is something in all of us that admires the Sir Galahad, who although flawed, is gallant. We look beyond ourselves and see a person who pursues a dream albeit flawed, a person whose "reach exceeds his grasp," as Lord Byron said. In this perception, our lives take on a bigger dimension, if only momentarily, and we vicariously feel larger in this act of romanticism.
Nick is clearly intrigued by Gatsby, but I think he also sees through Gatsby's dreams and is hesitant to allow himself to be too caught up by them. He remains as the outside observer, making commentary and coming along for the ride, standing firm as a solid support for Gatsby in a world that is populated by individuals who would use Gatsby for what he can provide but who are not true friends. I don;t believe that Nick is ever drawn into the fantasy completely. He sees it in much the same way we watch a movie or a car wreck - as outsiders but inserting ourselves peripherally into the story. Nick is, in fact, the only character who grows or is changed in any way as a result of having known Gatsby, even though circumstantially his life is the least impacted. He does not die (as do Myrtle, Wilson, and Gatsby) and he does not commit a crime as does Daisy, but he is changed for having seen the fault of falling victim to an unrealistic dream. Yet he does not negate dreamers. He still admires Gatsby's ability to have faith in his dreams, to live his dreams, even as he comes to realized the fact that those dreams were futile from the start.
You are quite right. In The Great Gatsby, Nick aligns himself with Gatsby, who is symbolic of the idealism and romanticism of the American Dream itself.
As one critic says:
Nick’s attitudes toward Gatsby and Gatsby’s story are ambivalent and contradictory. At times he seems to disapprove of Gatsby’s excesses and breaches of manners and ethics, but he also romanticizes and admires Gatsby, describing the events of the novel in a nostalgic and elegiac tone.
Nick as unreliable narrator who retells the story from home and uses the his authorial voice to construct himself. Although Nick claims objectivity to distance himself as dispassionate observer. But then, he virtually equates himself with Gatsby. Nick has a personal investment in Gatsby (and his own affection for Daisy). As such, Nick is denying his own sexuality; instead, he is living vicariously through Gatsby's.
Whereas Gatsby's desires are focused, specific (Daisy), Nick's are unfocused. Nick has unfocused desires: he desires to know, but when he gets close, it's too real, too messy. Nick is drawn to human secrecy, and his boring personal life is attempt to mask this fact.
Nick is a hypocritical moralist, just like all of us when it comes to our dreams, our families, friends, and our countries. We love them and we hate them, but to a stranger and enemy we will always, in the end, defend them. Especially after they are dead and gone.
To Nick, relationships are too much trouble -- one should curb fantasies, resist all strong connections. At least that's what his father told him. But then, along comes Gatsby, and Nick denies his father's advice and participates in Gatsby's "old sport" of trying to relive his boyhood. In short, Nick starts the novel in the superego, and then he is seduced by the id as soon as he meets Gatsby. Any moralistic attempts to deny otherwise is self-delusion.
In The Great Gatsby, Nick is an involved, first-person narrator and as such can be unreliable. He is judgmental and makes first impressions (which as a rule are usually unreliable). He also brings an outsider's view--Midwestern--to an Eastern situation. Thus, the possibility always exists that he is deluded.
In the case of his view of Gatsby, however, he seems well aware of Gatsby's shortcomings. He just likes him anyway. Nick is a modern narrator, and as such he doesn't expect people to be perfect. Gatsby is realistic in the sense that he is a mixture of many traits. To be deluded would be to see Gatsby as perfect. Nick doesn't see Gatsby as perfect--he just likes him anyway.
Gatsby is in love. Gatsby, in a sense, loves like all human beings should love--fully. Gatsby's love is all-consuming. He is the type of the legendary suitor who will do anything for a chance with the person he loves. And his vitality is thrown into that chase. Nick admires this.
Nick is not deluded, however. He says early in the novel that Gatsby is everything he hates. He tells the reader about Gatsby's connections with the mob. He explains, in so many words, that Gatsby's view of Daisy and of his relationship with her is an illusion. He knows Gatsby is somewhat foolish, and especially that he is foolish when he still thinks he has a chance on the final night of his life after Daisy has publicly rejected him and he waits outside of her house and waits for a phone call the next day.
He also makes clear to the reader that Gatsby is socially inept (just to provide another example of Nick's realistic view of Gatsby).
Gatsby is not presented as a perfect person by Nick. He is presented as possessing more faults than virtues. Yet, the one virtue he does possess fascinates Nick, as it probably should. Whatever else the novel is, it is first a tragic love story.
As sophisticated postmodern readers, out job today is more to understand a fictional character, than it is to judge him or her. Fictional characters are not perfect--not in serious literature, anyway. But there is something of the human condition in them.
I think Nick struggles with wrapping his mind around Gatsby's dream throughout the novel. At times he admires Gatsby. The more he gets to know Gatsby, the more he realizes how driven Gatsby is to win Daisy, the more he realizes how much Gatsby has overcome to get even as close as he does get to winning Daisy. Nick is impressed by the colossal vitality of Gatsby - the way he goes overboard in everything he does, his parties, his house, his clothes, his cars. Gatsby is driven to give 200% to achieve his goal because his beginnings were so humble. He is nouveau riche, but Daisy is Old Money. That is a gap that ultimately cannot be bridged and, as Nick realizes in the end, Gatsby never really had a chance of bridging that gap because "Tom and Daisy were careless people" and "smashed up things and creatures" before retreating back into their money.
The final lines of the novel, though, still illustrate your point, that Nick is still charmed by Gatsby's dream, even though it was futile. It was the act of having the dream in the first place that was admirable to Nick:
Gatsby believed in the green light, the orgastic future that year by year recedes before us. It eluded us then, but that’s no matter—to-morrow we will run faster, stretch out our arms farther. . . . And one fine morning——
I believe this shows that Nick believes that Gatsby's dream was a noble one, a charming one, even if a futile one. I don't think he is deluded to admire Gatsby for having the dream even though the dream is what destroyed him.
This is my personal opinion - you will find it differs somewhat from some other commentary on the novel but hey, that's what makes good literature, right?
I assume that you are saying that Nick is deluding himself into thinking highly of Gatsby -- that Nick is fooling himself when he believes that Gatsby was a good, admirable person.
If that's what you are saying, I'm willing to go along with you. I do not think that Gatsby is admirable. I think that he does have this colossal vitality that he uses to pursue Daisy. But that doesn't make him good. He seems shallow to me -- he has no real goal in life other than to have Daisy. To me, if he were truly admirable, he would have gotten rich in a more legal way and he would have a better reason for getting rich.
So I think Gatsby is shallow and maybe a bit dumb. However, he has a lot of energy and at least he wants to be rich for a reason, however dumb the reason is. I think that is why Nick likes him.