In Chapter 6, why does Fitzgerald reveal the truth about Gatby's background at this particular point in the novel?

In Chapter Six, Nick advises Gatsby not to ask too much of Daisy, not to ask her to repudiate her love for Tom:  “You can’t repeat the past,” he says. Gatsby disagrees:  “Why of course you can!” (116). Even the exclamation mark is significant here because it indicates the enthusiasm and certainty of Gatsby’s response. His life since he lost Daisy in his youth has been directed to this moment of being able to “get her back” so that he can relive the dream he had made her into when they first met, captured especially in their first kiss. Fitzgerald tells us about Gatsby’s past at this particular moment because doing so coincides with Gatsby’s opportunity to recreate it. In other words, Fitzgerald and his character recreate the past together, the author by the structure of his novel, and the character by the life he has led and the dreams he has nurtured up to this point.