Crooks is ostracized by the other men because of his race. He has to live by himself in a little room off the barn. It isn't even his room but a place where they store equipment for the horses. It smells bad because of the manure inside and outside. Although he is in constant pain from his injured back, he has to sleep in a long box filled with straw. Crooks is bitterly resentful, but he pretends to be indifferent. He is a proud man. Since nobody comes to visit him anyway, he pretends that he doesn't want any visitors. Lennie doesn't understand prejudice and discrimination. He is just looking for company when he intrudes on Crooks, who tells him:
"You ain't go no right to come in my room. This here's my room. Nobody got any right in here but me."
Crooks is rude and even cruel to Lennie because Crooks needs to take his anger and hurt feelings out on someone, and poor Lennie has been the target of much abuse over the years because of his mental impairment. Even George subjects Lennie to verbal abuse from time to time, as he did at the riverside campsite in the opening chapter. But Crooks goes too far when he starts suggesting that George may have abandoned Lennie when he went into Soledad and might have no intention of coming back. Lennie does not understand that Crooks is only speaking hypothetically, and Crooks has to backtrack because Lennie is becoming dangerously suspicious and potentially violent.
Crooks saw the danger as it approached him. He edged back on his bunk to get out of the way. "I was just supposin'," he said. "George ain't hurt. He's all right. He'll be back all right."
This is only one of the many conflicts that lend drama to each chapter in Of Mice and Men. The chapter in which Crooks is rude to Lennie also contains conflicts with other characters--Candy, Curley's wife, and finally George-- who intrude because Lennie seems to have opened up Crooks' room to visitors. Crooks is relieved to get rid of them all and go back to his peaceful, solitary existence.