Chapter Four of Steinbeck's Of Mice and Men focuses on the fringe characters (those isolated and alienated by society): Crooks, Lennie, Candy, and Curley's Wife.
The setting takes place in the barn, where Crooks keeps a room and a small library. Lennie obviously doesn't understand the social segregation here, as he invades on Crooks' privacy. Ironically, Crooks is upset that he doesn't practice social segregation. Also ironic is the fact that, even though it is near the animals, Crook's room is a place of learning, unlike the place of violence that is the bunkhouse and the place of sex that is the cathouse.
Crooks is lowest on the social-Darwinian ladder than even Lennie, and he seems to have gotten used to not socializing. Unlike Candy, who buys into Lennie's American pipe dream of owning the farm, Crooks immediately sees through it, calling it crazy. He even pokes fun at the companionship that George and Lennie have fostered, which angers Lennie. So says Enotes:
After a while Crooks decides that it is safe to talk to Lennie, since Lennie is obviously “crazy as a wedge.” He tells Lennie about his childhood, revealing his days as a boy on a chicken ranch, playing with the white kids. He gets excited about the idea of having someone to listen to him. But Lennie doesn’t understand him, and he isn’t even listening. He is more concerned about the puppies in the barn and the rabbits they’re going to get. Envious of Lennie’s relationship with George, Crooks teases and torments him. He asks Lennie to imagine that George has left him for good, that he got hurt, and that Lennie will never see his friend George again. Crooks takes pleasure in his torture of the frightened Lennie. “Want me ta tell ya what’ll happen?” he asks Lennie. “They’ll take ya to the booby hatch. They’ll tie ya up with a collar, like a dog.” But when he sees the danger of upsetting Lennie, he reassures him that George will return.
Like Candy, Crooks is physically wounded. Like Curley's wife, Crooks is socio-economically wounded. So, Crooks has the double-whammy of the American "dream" working against him.
Each character in the novella is analogous to an animal, and Crooks' is a mule. He was kicked by one, and he lives near them. He is treated like an animal, and he stubbornly acquiesces so that he may be left alone with his book apart from the corrupt and violent white society.