Why does Gatsby believe that he can go back to Louisville?
Gatsby thinks that he can make things just like they were in Louisville with Daisy all those years ago because he believes that it is possible to repeat the past. When Nick tells him that you can't relive the past, Gatsby cries, "Why of course you can!" Gatsby thinks that it is absolutely possible to recreate the way things used to be, the way people used to feel, because he so badly wants it to be true. Nick says,
"[Gatsby's] life had been confused and disordered since then, but if he could return to a certain starting place and go over it all slowly, he could find out what that thing was...."
In other words, Gatsby's life since he was in Louisville with Daisy has not gone the way he would have liked it to; there has been war and chaos, and the loss of her, and he vainly hopes that in somehow going back to the point before it all went wrong, he will be able to recover everything that he's lost and prevent his losing it again. This is just part of Gatsby's impossible dream though because, as Nick says, it really is impossible to repeat the past, especially when Daisy is now a wife and mother and simply cannot make the same kinds of decisions she would have made then.