From Ch. 1, when Daisy responds to the mention of Gatsby's name with, "Gatsby? What Gatsby?" in such a way that the reader knows she knows this name until Ch. 8, the reader has been given bits and pieces of the former relationship between Jay and Daisy. In Ch. 4, Jordan tells Nick about the day Daisy married Tom and how Daisy had received a letter which caused her to get drunk and attempt to call off the wedding. It's clear that this letter was from Jay - but again, the reader is only given a glimpse of the past relationship. Still, the reader is slowly piecing together the puzzle of Daisy and Jay. Ch. 5 gives readers another piece when the truth comes out that Jay's money has been earned only recently and that Jay longs to return to the days in Louisville, five years earlier. In Ch. 6, the reader learns of the first kiss between Jay and Daisy and how Jay fell in love with Daisy and knew then that she was essential to his life. Ch. 7 is where the reader is taken from the romantic memories to the harsh reality when Jay and Tom nearly come to blows over Daisy. By the end of Ch. 8 Jay Gatsby is dead. The reader has to have the final pieces to this picture puzzle to see completely the relationship between Daisy and Jay, so now Fitzgerald gives the reader those final pieces.
At the beginning of chapter 8, Gaysby explains to Nick his initial love affair with Daisy. Fitzgerald choose to have this explained to the reader to add a dramtic effect; now we know why Gatsby goes through so much trouble in order to impress Daisy. It explains the reason behind his life long American Dream.