Tom and Daisy in "The Great Gatsby" reconcile because of the downfall of Jay Gatsby's persona. After Tom explains to Daisy who Gatsby really is, a dishonest bootlegger, Gatsby loses his temper. It is at that point that the idea of who Jay Gatsby is begins to crumble, making Daisy return to her husband, Tom.
Daisy Buchanan was always attracted to money. It is because of this that she marries Tom Buchanan despite still loving Jay Gatsby, a poor soldier. Gatsby spends five years building up a fortune and a property and hosting elaborate parties to attract Daisy to come back to him. He ends up succeeding for a period of time. Daisy finds that Gatsby is the loving gentleman she wants to take care of her instead of her adulterous husband. When Daisy realizes that Gatsby is not really who he says he is, which occurs because of Tom's background search of Gatsby, she decides that she can no longer be with Gatsby. She and Tom slowly reconcile. Tom makes sure that Gatsby is never in Daisy's life again by telling George Wilson that Gatsby is the one who killed Wilson's wife, who also happens to be Tom's mistress. With Gatsby completely out of the picture, he and Daisy are able to move on with their lives as a married couple. They return to their "secret society" of being careless and not worrying about whose lives they endanger along the way.