Why did Crooks change his mind after Curley's wife left the bunkhouse?In chapter 4
Crooks's hopes begin to die when Curley's wife lashes out at him in retaliation after his vehement self-assertion in the face of her insults to him, Candy, and Lennie. Later, when George returns from town and enters the barn, Crooks is discouraged more.
After entering the barn univited, Curley's wife boldly derogates Crooks, Lennie, and Candy, calling them "a n----r, a dum-dum" and a "lousy ol' sheep." Careful around a white woman, Crooks "retire[s] into the terrible protective dignity of the negro." But when Curley's wife continues her ridicule of the three men who have befriended him and included him in their dream of owning a farm, Crooks suddenly stands up, angrily kicks the nail keg on which he has been sitting, and comes to the defense of the men who have befriended him.
"I had enough," he said angrily. "You ain't wanted here. We told you you ain't. An' I tell you, you got floozy ideas about what us guys amount to. You ain't got sense enough in that chicken head to see that we ain't stiffs. Suppose you get us canned....You don't know that we got our own ranch to go to, an' our own house....An' we got frens', that's what we got....We got our own lan', and it's ours, an' we c'n go to it."
Hearing Crooks's forceful assertions in which he includes himself with Lennie and Candy, Curley's wife reacts in typical fashion for the time period, the Jim Crow 1930s. Turning on Crooks "in scorn," she threatens him with hanging if he does not "keep his place." In a toneless voice this time, Crooks replies in submission: "Yes ma'am." Candy comes to his defense and declares that he and the others would tell the men that she "framed Crooks" if she carries out this threat. Confident, Curley's wife asserts that no one would believe them. The old man concedes, "No. . . . Nobody'd listen to us." However, Candy then cleverly suggests that the woman return home because he has heard the men who have gone to town now return to the bunkhouse. "You better go home. . . . If you go right now, we won't tell Curley you was here." She does not believe that he has heard anything, but a clever Candy quietly says, "better not take no chances."
After Curley's wife departs, Crooks asks Candy if he has heard the men, because he has heard nothing. "The gate banged" is all Candy replies. He tells Crooks that the woman should not have talked to him as she did, but Crooks simply replies "dully," "It wasn't nothing....You guys comin' in an' settin' made me forget. What she says is true." Not long after this, George enters and finds Lennie in Crooks's room. As proof of what Crooks has just said, "[George] looked disapprovingly about. 'What you doin' in Crooks' room. You hadn't ought to be in here.'" This comment of George's seals the despair that Crooks feels about any hope of joining the others in the dream of owning a ranch. In another act of the "protective dignity of the negro," Crooks calls out to Candy and retracts his offer to do odd jobs and hoe on the dream farm. By acting as though he does not want to join the others, he saves himself the embarrassment of the likelihood of George's rejection after Candy is scolded for telling him about their dream. Saying "I wouldn' want to go no place like that" is his way of retaining some pride.
In the novel 'Of Mice and Men' by John Steinbeck, Curly's wife illustrates one of the themes - power - when she has the cheek, front and sheer nerve to invade a person's privacy and personal space. She should not be anywhere near the bunkhouse or rooms and has no proper business there. Yet she flaunts her power over poor black Crooks by reminding him of his hopeless station in life, and that there is nothing he can do about her actions. She exploits his powerlessness to do anything about her invasion by threatening him with 'framing him' for the very sorts of misdemeanours that Lennie Small is actually guilty of. She has the power of color and staus - even though she is a woman. Crooks remembers why they are all where they are.
To me, the reason that Crooks changes his mind is because Curley's wife has robbed him of all hope for the future.
Before Curley's wife comes into the bunkhouse, Crooks has started to buy in to the idea of going along and chasing the dream that George and Lennie have been chasing. He is finally feeling some hope.
But then Curley's wife comes in and smashes that. She forcibly reminds him about how powerless he is because of the fact that he is black. She reminds him that she could have him lynched just by saying he touched her.
So now Crooks is demoralized. He feels like he is nothing and no longer has the ability to dream of a better life.