In The Great Gatsby, Jay Gatsby is basically broke when he meets Daisy. When he goes off to fight in the war, he vows not to lose her because of money. He gets out of the army, goes to college and then gets involved in a drug ring and organized crime. He does this solely to build himself up in order to win over Daisy. Since Daisy is a popular, beautiful girl from a wealthy family, Gatsby felt it was necessary to gain wealth to be in that same social class.
When Gatsby and Daisy fell in love, prior to her marriage to Tom, Gatsby recognized that Daisy was on a higher social or economic bracket than he was. And although Gatsby had always put Daisy on a pedestal, he played it like he was a part of that same upper class. Nick gives us some insight in Chapter 8:
I don’t mean that he had traded on his phantom millions, but he had deliberately given Daisy a sense of security; he let her believe that he was a person from much the same stratum as herself—that he was fully able to take care of her. As a matter of fact, he had no such facilities -.
In fact, obtaining wealth was just one part of making himself, really recreating himself, into a more well rounded person. Gatsby's father comments on this in Chapter 9. "Jimmy was bound to get ahead. He always had some resolves like this or something. Do you notice what he’s got about improving his mind? He was always great for that." Mr. Gatz tells Nick this after showing one of Jimmy's (Gatsby's) old regimens which included working out, reading, elocution and studying various subjects. Gatsby was always positioning himself towards some goal (Nick refers to it as the "following of a grail"). Gatsby felt he needed wealth to be a good match for Daisy. Acquiring this wealth was just one aspect of remaking himself into someone he thought would be more socially acceptable as Daisy's husband (in Daisy's eyes and society's) .