Here is the end... Where is the Theme?

Reading the last chapter of The Great Gatsby by F. Scott Fitzgerald, I can't help but notice a lot of irony.  Most of it is in the fact that Nick, who at the beginning of the story said that Gatsby represented all that he resented, is the one who worries about him after his death.  He says "I found myself on Gatsby's side, and alone (164)."  He even claims to have been close friends with Gatsby when talking to Mr. Gatz.  While Gatsby's death was truly tragic, the only things that Gatsby gave Nick were problems.  Why should Nick spend even more time arranging Gatsby's funeral and trying to find non-existent friends to attend.  Again I see Nick as a bit of a pushover, for he didn't truly have to do anything for Gatsby.   On a completely different note, I'm not entirely sure what the theme is.  Randomly at the end of the novel, Nick relates Gatsby to "us" saying "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.... So we beat on, boats against the current, borne back carelessly into the past (180)."  I guess Fitzgerald is trying to warn people against spending life pursuing an unlikely goal, what would for most people be fame or wealth.  In that sense, Gatsby and his pursuit of wealth and social standing through Daisy would symbolize all people who strive for something unlikely or unnecessary.  However, because Fitzgerald uses the word "we," I wonder if he is referring to a whole group of people,  most likely American citizens of the 1920's.  In the "Roaring 20's," a time of great economic prosperity, technological advancement, and material wealth, Fitzgerald could be warning Americans not to become too caught up in the American Dream.  He wants them to rethink their priorities, placing less importance on pleasure, fame, and money.  If this is true, his warning was very smart, for America soon plunged into the Great Depression, and the period of glamour and wealth vanished.