All of Jay Gatsby's hopes and dreams are depicted for readers through the critical eyes of Nick Carroway. Nick admires the purity, intensity, resilience, and strength of Gatsby's obsession with Daisy and the materialistic, acquisitive ambition required to win Daisy. To possess Daisy and her Old Money world, Gatsby is willing to remake his own identity, work ceaselessly and illegally with the likes of Meyer Wolfshiem, take the fall for a hit-and-run accident, stand guard all night outside of her house, and wait obsessively for a call that every reader knows will never come. In Nick's opinion, the sacrifices that Gatsby makes for his ambition of claiming Daisy border on obsession; however, through Nick, the novel hints that even the most intense and absurd obsessions contain admirable and rare glimpses into the human capacity for persistence, hope, imagination, and love.
The whole book from one perspective is about obsession. If we ask why Jay Gatsby did what he did, we can say that he did everything for Daisy. He did whatever it took to become rich, even if he had to pursue unsavory means to gain riches. He reinvented his persona to become what he thought Daisy would love. His dress and manner of speaking and acting were all affectations. He also threw great parties that were beyond extravagant in the hopes of catching her attention or even getting a glimpse of her. Finally, Jay takes the blame for Daisy when Daisy hit Myrtle in a car accident.
We can, of course, say that all of this was motivated by love, but it becomes increasingly clear to the reader that Jay would not succeed. The harder he worked to gain Daisy, the more he lost her. Obsession and ambition made Jay blind. He could not see what was clear to others. From another perspective, we can say that Jay was the best man in the book, because at least his obsession and ambition were devoted to something noble - the love of Daisy.