Jay tells Nick that Daisy was the first girl he knew who wasn't one of loose or questionable morals. The girls and women Gatsby had known were ones who were easy for him to get and easy for him to just as quickly discard. He didn't intend to fall in love with Daisy, but it happened and Gatsby found himself committed to her absolutely and for everafter. In all his previous relationships, it was Gatsby who made the moves and the decisions. He was in charge of the affair. With Daisy, he soon found that he was not in charge. Daisy was everything, and more, that Gatsby had ever dreamed of. She was rich, young, beautiful, vivacious, exciting, and mysterious. And she loved Gatsby, too. Jay also tells Nick of how he, Jay, had to leave Daisy because he had to go overseas during the war. They exchanged letters, but hers were mostly about how she wished he'd come home and soon, she grew tired of waiting, and Tom Buchanan came into her life with lots of excitement and pomp. Add to that the fact that Jay had no money and was from a completely different social class. Jay tries to rationalize Daisy's relationship with Tom in this conversation with Nick. He is trying to rationalize it to himself more than to Nick. He tells Nick all this because Jay is seeing his fragile world start to crumble. After the big scene in the hotel in NYC, a part of Jay must realize that he will never fully get Daisy and that their affair is probably over. Jay wants to hang on to whatever scraps remain of his illusion of a world where he and Daisy are a couple and retelling the story of his and Daisy's courtship helps Jay to hang on.