Why does Gatsby turn the clock back five years?

In The Great Gatsby, Gatsby does not literally turn the clock back five years, but rather does so figuratively with Daisy. At the end of Chapter 6, Nick Carraway, the narrator, ends the chapter with a description of how Gatsby and Daisy's love first blossomed five years ago. In that same area, when Nick warns Gatsby about repeating the past, Gatsby incredulous and says a famous line "Can't repeat the past?...Why of course you can!" (Fitzgerald 110).

Gatsby, who even purchases a house across the Long Island Sound from Daisy Buchanan's home just so he can see the green light on her house's dock every night, is very much in love with Daisy. The height of their relationship was five years ago when they first fell in love while Gatsby was stationed as an army lieutenant in Daisy's town. Gatsby wants to turn back time to when they were in love and Daisy was his sweetheart. Everything Gatsby does in this novel, is to bring him and Daisy closer together. He has the parties in hope that Daisy will one day show up to one of them. He has a house across from hers, just to see the green light on her dock every night.

As shown in the quote I cited above, Gatsby wants to (figuratively) turn the clock back five years because he wants the love that he and Daisy shared to blossom yet again.

My edition of the book: F. Scott Fitzgerald. The Great Gatsby. New York: Scribner, 1925.