George says this because Lennie will want to sleep with his puppy.
George said, "It wasn’t much to you, maybe, but it was a hell of alot to him. Jesus Christ, I don’t know how we’re gonna get him to sleep in here. He’ll want to sleep right out in the barn with ‘em. We’ll have trouble keepin’ him from getting right in the box with them pups."
credit goes to jill d #170087 http://www.gradesaver.com/of-mice-and-men/q-and-a/why-does-george-say-lennie-would-want-to-sleep-in-the-barn-that-friday-night--235180 Of Mice and Men...