In chapter 6, Nick recounts Gatsby's past. The reader discovers that his real name was James Gatz and that he had no use for his real parents. In fact, his imagination leads him to believe that he was "a son of God—a phrase which, if it means anything, means just that—and he must be about His Father’s business, the service of a vast, vulgar, and meretricious beauty" (chapter 6). In this recollection, Gatsby's fantastic imagination conjures up many dreams during his sleepless nights, a vast American Dream that consists of wealth and renown.
However, at the end of the chapter when he kisses Daisy, Gatsby realizes that she has become his ultimate goal, and "his mind would never romp again like the mind of God" (chapter 6). At the touch of their lips, she forever changes Gatsby's destiny. Daisy becomes Gatsby's ruin because she can truly never love him. She is interested in social status and the charm of old money. Gatsby's new money intrigues her, but she believes that new money is gaudy and crass even though she enjoys his lavish lifestyle.
Gatsby is wrapped up in his dream of Daisy, the Daisy he creates in his imagination starting from their first kiss. As the reader later discovers, Daisy is not innocent and pure. When Daisy kills Myrtle Wilson and abandons Gatsby at the end to remain with Tom, it is apparent Gatsby's focus on his silent vow to make her his own one day has distanced him from the reality of the situation. The American Dream that he pursues so relentlessly is as empty and as meaningless as Daisy's way of living.
At the end of Chapter 6, Fitzgerald writes, "He [Gatsby] knew that when he kissed this girl, and forever wed his unutterable visions to her perishable breath, his mind would never romp again like the mind of God." For Gatsby, kissing Daisy is like marrying her; he is vowing to forever pine for her and live for her from that day on. Chapter 8 also gives a sense of this vow when Fitzgerald writes, "He [Gatsby] felt married to her..." This vow would explain why Gatsby pursues Daisy relentlessly throughout the book; however, unfortunately for Gatsby, Daisy does not feel the same way for Gatsby that he feels for her. For Daisy, "She wanted her life shaped now, immediately--and the decision must be made by some force--of love, of money, of unquestionable practicality--that was close at hand" (Ch. 8). When Gatsby left for WWI, Daisy was left alone, so when Tom Buchanan showed up, she had that force she was looking for, thus she settled for Tom.