Short answer: Crooks asks the men to leave because Curley’s wife has just reminded him—through insult and threat—of his powerless, vulnerable position on the ranch. After briefly opening up and letting others in, he retreats into self-protection, shame, and the old social rules that isolate him.
Step-by-step explanation:
- He had been opening up. Crooks begins the scene defensive and alone, then talks honestly about his loneliness. He allows Lennie (and later Candy) into his room and even entertains the idea of joining their dream. That vulnerability is significant because Crooks normally keeps people out.
- Curley’s wife shatters that fragile trust. When she enters she insults and humiliates Crooks and reminds him of the real social dangers he faces as the only black man on the ranch. Her words make clear she can use her position to harm him.
- There is an explicit threat. She tells him, in effect, that she could have him punished or worse. Her power to destroy him is implied by lines like her threat that she could have him ''strung up on a tree so easy it ain't even funny.'' That threat turns the moment from a private, consoling conversation into one of real risk.
- Crooks retreats to protect himself. Having been exposed and humiliated, Crooks reasserts the boundaries that keep him safe (if lonely). Asking the men to leave is a way to close off that vulnerability: better to be isolated than to risk further injury or exposure.
- He may also be protecting the others and the dream. The dream of the little farm is fragile. Crooks knows that associations and talk can bring trouble; by pushing the others out he reduces the chance that Curley’s wife’s threats will lead to wider consequences for them or their hope.
- This moment reinforces Steinbeck’s themes. The scene underlines how racism and power keep people isolated. Crooks’ quick swing from briefly imagining connection to insisting on solitude shows how social hierarchies destroy intimacy and hope.
Two lines from the chapter help show this emotional arc: Crooks’ admission of loneliness — ''A guy goes nuts if he ain't got nobody'' — and the cruel threat from Curley’s wife about how easily she could have him punished. After her exit, Crooks chooses the familiar shield of distance instead of risking further hurt.
In short: Crooks tells them to leave because Curley’s wife exposed the danger of intimacy for him. He protects himself (and perhaps the others) by returning to isolation, showing how social power and racism crush the possibility of shared hope.