The failings of nearly every character in the novel can be taken as being depressing. There is a significant lack of moral integrity in the novel's characters. Even Gatsby, who Nick suggests is worth the whole lot of the rest of them, even he is a liar, a criminal, an adulterer and a snob.
However, there is a sense that the innocence with which Gatsby undertakes to build his wealth and to create his persona remains intact from start to finish. Despite his failings, Gatsby represents something higher, something almost pure. Nick recognizes this and I take Fitzgerald's view to be related to Nick's on this point: even those who we easily condemn have pure dreams.
What bothers me most emotionally about Gatsby is the idea of the careless prospering. Nick's famous thought at the novel's end about Tom and Daisy being careless people who smash up others' lives and then retreat into their cocoon of wealth has always profoundly affected me. I want punishment for the Buchanans. I want them to be prevented from smashing up someone else's life. Perhaps this aspect of the novel disturbs me so much because it is still realistic. How many times do we hear of a careless drunk driver walking away from a wreck that killed innocent, unsuspecting passengers in another vehicle? How many times do we read about someone's careless words causing another person a lifetime of grief?
The satiric tone of much of the novel presents the sordid truth about America that is populated with jaded, materialistic people whose greatest goal is money and all that it can buy. There is little spirituality in any but Jay Gatsby, the novel's tragic hero.
The book can be read as a commentary on the American Dream, and while we are all told that "if you work hard enough" you can achieve your dreams, this novel makes it clear that you can't. No amount of hard work or perseverance is a guarantee of anything, much less, health, wealth, and happiness.
To me, it's depressing because the characters are either hard to like or bad things happen to them. Nick's fine, but he's not really part of the story, to me. Gatsby's sort of an idiot for wanting Daisy, but you do sort of like him and hope things will turn out for him. But they don't. He gets killed because Daisy totally betrays him. Then she and Tom go off together and nothing bad happens to them.
So there's no satisfaction, no joy to the book. The bad guys do fine and the good guys (to the extent that there are any good guys) don't. That's depressing in my mind.
In large part, there is a sense of sadness in reading Fitzgerald's work because there is no sense of redemption present for the characters in the novel. Fitzgerald is seeking to bring out what the condition of the time period is like. In this light, he is articulating the extreme hollowness that is present in the Jazz Age. Underneath the veneer of classy clothes, great parties, ornate homes, and bedazzled flappers lies a corroded setting, where individuals are used as means to ends as opposed to ends of themselves. Additionally, there is little moral or ethical structure that is governing the manner in which individuals interact with one another. In a political setting of deregulation and lack of authorial interference, there seems to be an emotional paralell whereby there is little structure guiding individuals and the manner in which they behave. The Toms of the world are going unpunished for their brutish and intensely uncaring ways, while the Jay Gatsbys of the world are suffering for their transgressions. Amidst all of this, the Daisys and Jordans are simply looking for the next party and the next kernel of gossip. There seems to be little in way of moral structure, and even less in way of redemption. Nick's departure from this condition might be seen as empowering, but really it is nothing more than a flight and abandonment of hope.