The book's ending shows that nothing meant anything. All of the drama and chaos, all of the loss and heartbreak, and everything still just keeps going. Nick experienced the whole thing, but it is time to move on.
I have always been struck by how well Fitzgerald shows that life simply goes on, even after someone like Gatsby dies. There is a certain normalcy that Fitzgerald's writing evokes at the beginning and end, perhaps achieved best by his employment of Nick as the narrator, an individual who stands apart from the other characters; even after developing a relationship with them and spending time with all of them in their "lifestyle", Nick still remains a kind of third party observer for the reader. Fitzgerald brings the novel full circle with Nick contemplating his father's advice at the beginning and again at the end (as stated in the previous posting), advice that is wise, yet seems rather practical and emphasizes the need for a detachment from the judgmental ways of the other characters, all of which again stresses the importance of normalcy.
This is a personal opinion question, but I believe it is interesting that the book opens and closes with Nick, the narrator, contemplating his father's advice concerning people who haven't had the advantages he (Nick) had as a youth. At the close of the book, Nick contemplates Gatsby's life, his belief in "the green light." Gatsby did not have the privilege that Nick grew up with but was fueled by a dream of the future. This leads to an interesting concept in both the opening and closing of the work: What role does the motif of living for the future play?
I found the ending quote very interesting. "So we beat on, Borne back ceaselessly into the past" (189). No matter what the characters do, their attempts to push forward in class are futile due to the ever present effects of the past. Myrtle and Gatsby cannot move forward due the way they grew up and the lack of support from an old money family.