In the denouement of John Steinbeck's Of Mice and Men, George has Lennie repeat the dream for two reasons:
First of all, Lennie loves the dream and delights in its repetition. It is a calming force for Lennie, and one that brings him joy. So, George gives his childlike friend the last and final treat before sending him to heaven.
Secondly, Lennie is the keeper of the dream. George has always known that they will never realize the dream of having a ranch of their own, but he has half believed in the dream by the prayer that it has become through repetition. But, since George now realizes that the dream cannot exist without Lennie, he ends the dream as a final prayer with Lennie's death.
Towards the end of Chapter 6, George is about to kill Lennie. His last words to Lennie have to do with their dream. He tells the whole story to Lennie again -- how they will live, what it will be like. Then he kills Lennie.
The reason he repeats this is because that is what Lennie wants from him. Lennie wants to be reassured that their dream is still alive even after he has killed Curley's wife. So George tells him the story of the dream and that makes it so that Lennie dies happy. He is thinking about their dream life and George shoots him -- Lennie never knows that he's about to die.
Right before George shoots Lennie in the back of the head to spare him a terrible death, Lennie asks George if he can recite their dream. George proceeds to repeat the story about how they will own a piece of property where they will live together and raise several animals. George decides to repeat the story for several reasons. George is close friends with Lennie and is simply giving Lennie his last request. Lennie's last request is to have George tell the story, and George sympathetically repeats the story to please his friend. George also knows that the story will calm Lennie down which will allow him enough time to hold the gun up to Lennie's head without him noticing. Also, George repeats the story before he kills Lennie to symbolically represent how their dream will never be fulfilled. Knowing that their dream will never be realized, George repeats the story to comfort his friend before his death.