Crooks has a room to himself because he is antisocial by nature and doesn't get along well with the other men. He has been working at the farm longer than most of them and has "earned" this meagre privilege.
Another reason is that he is a Negro and is therefore naturally excluded from the farmhands' circle. Also, since his accident, Crooks is no longer able to work out in the fields doing "a man's work" but is delegated domestic tasks about the farmhouse. Both his colour and his handicap ostracize him as an outsider, and he feels bitter about this difference.
Crooks takes refuge in his room, and the few belongings he does have show that he is more intelligent than he appears to be. For instance, he has books he cherishes and is particularly protectionalist when Lennie comes by the evening the other men have gone out to town. When he sees Lennie means no harm, he says he can stay, then confesses about how lonely he really is. Underneath the "sour grapes treatment," Crooks is longing for the acceptance and companionship he is denied.
In John Steinbeck’s novel Of Mice and Men, the character of Crooks is an old, cranky ranch hand with a seriously bent back resulting from years of hard manual labor. He is initially unfriendly to Lennie, as he doesn’t yet know of the giant’s gentle nature and mental deficiency. He has his own room because of his generally anti-social attitude, but it is an attitude born of the second-class status to which African Americans were condemned. As Steinbeck notes, “Crooks was a proud, aloof man. He kept his distance and demanded that other people keep theirs. . .” He is segregated from the other ranch-hands because of his racial distinction relative to the others, so has his own room, which he keeps clean and organized. When the innocent simple-minded Lennie stands in the doorway of Crooks’ room, the old, wounded African American angrily confronts this interloper:
“You go on get outa my room. I ain’t wanted in the bunk house, and you ain’t wanted in my room.”
“Why ain’t you wanted?” Lennie asked.
“’Cause I’m black. They play cards in there, but I can’t play because I’m black. They say I stink. Well, I tell you, you all of you stink to me.”
Crooks, however, is no simple-minded man. On the contrary, he is educated and his room is populated by old magazines and books, including a dictionary and “a mauled copy of the California civil code for 1905.”
A good description of Crooks’ room come straight from Steinbeck’s novel:
“Crooks’ bunk was a long box filled with straw, on which his blankets were flung. On the wall by the window there were pegs on which hung broken harness in process of being mended; strips of new leather; and under the window itself a little bench for leather-working tools, curved knives and needles and balls of linen thread, and a small hand riveter. On pegs were also pieces of harness, a split collar with the horsehair stuffing sticking out, a broken hame, and a trace chain with its leather covering split. Crooks had his apple box over his bunk, and in it a range of medicine bottles, both for himself and for the horses.”
Crooks is a blacksmith, and his life is the welfare of the horses that populate the ranch. His is a lonely existence, and Lennie’s presence, while initially unwelcome, becomes a reminder to Crooks of the importance, and fragility, of human relationships.