In chapter 4, Lennie is talking to Crooks about the rabbit ranch that he and George are going to have, but Crooks suggests George may not come back.
Lennie wanders into Crooks’s room and talks to him about the rabbits. George has been telling Lennie that they are going to have a ranch someday.
Lennie has come into the barn because he wanted to see his puppy. Crooks tells him he is not wanted, and Lennie whines about the others going into town and leaving him behind to stay out of trouble. Crooks asks where everyone is, and Lennie tells him that everyone but Candy went into town and Candy is figuring about the rabbits.
"The rabbits we're gonna get, and I get to tend 'em, cut grass an' give 'em water, an' like that." (ch 4, p. 34)
Lennie is excited because the rabbits are a dream that George and Lennie use to keep them going. Whenever George wants to focus Lennie or cheer him up, he talks about the rabbits. Lennie gets excited about caring for rabbits, because they are soft.
Crooks is annoyed that Lennie has someone, and he does not. He tells Lennie that George may not come back.
"He won't do it," Lennie cried. "George wouldn't do nothing like that. I been with George a long a time. He'll come back tonight-"But the doubt was too much for him. "Don't you think he will?" (ch 4, p. 35)
The idea that George might leave him and never come back is terrifying for Lennie. He depends on George for protection and almost everything else. He gets excited when he thinks about the possibility of George leaving him, because he has not considered it before. He seems to think that it is not possible at first, but then doubt creeps in and he gets worried.