Why does Lennie become upset with Crooks in Of Mice and Men?
Realizing that he has the intellectual advantage, Crooks tries to rile Lennie, who is looking for George, by telling him that George is not returning. He upsets Lennie simply because he can; this behavior of Crooks demonstrates the tendency of men who are too often alone to become mean and resentful.
Thematic of Steinbeck's Of Mice and Men is the Fraternity of Men that provides individuals strength and companionship. Without this fraternity men become mean and defensive, and in their alienation they lose meaning in their lives, because meaning comes from sharing. And, so, there is another reason that Crooks taunts Lennie by saying that George may not return from the whore house in town.
Crooks' face lighted with pleasure in his torture...."Le's say he wants to come back and can't. S'pose he gets killed or hurt so he can't come back."
Lennie struggled to understand. "George won't do nothing like that...he's careful....Say, what you doin' anyways?....This ain't true. George ain't got hurt."
Crooks bored in on him."Want me ta tell ya what'll happen?They'll take ya to the booby hatch. They'll tie ya up with a collar, like a dog."
When Lennie becomes threatening, Crooks backs off, and he reassures Lennie that George is all right. Lennie "growled back," Crooks, then, asks him, "Maybe you can see now," meaning now, perhaps, Lennie understands what it is to be alone. Crooks tells Lennie that a man needs someone to be near him.
"A guy goes nuts if he ain't got nobody....I tell ya a guy gets too lonely an' he gets sick."
This companionship helps a man know what he thinks makes sense, or what he sees is really there. There is a necessity for man to have others around him, to have someone against whom he can measure himself. The fraternity of men, the dream of living together and looking out for one another, is thematic of the narrative, and nowhere is it brought to point than in the interchange between Lennie and Crooks in his room.