One of the main themes of John Steinbeck is the disfranchisement and the consequent alienation of those in the lower classes. Both his magnum opus, The Grapes of Wrath and his novella Of Mice and Men present this theme of alienation.
This loneliness of the itinerant men in Of Mice and Men generates cruelty and injustice in others as exemplified in the characters of Curley and Carlson. George Milton expresses this reality as he talks to the man with "God-like" eyes, Slim, the mule skinner:
"I seen the guys that go around on the ranches alone. That ain't no good. they don't have no fun. After a long time they get mean. They get wantin' to fight all the time."
"Yeah, they get mean," Slim agreed. "They get so they don't want to talk to nobody."
Much of this aggression is also generated by fear in their aloneness. Lennie Small, whose character Steinbeck wrote represented "the inarticulate and powerful yearning of all men" certainly becomes aggressive in his fear.
It is only through the fraternity of men, through friendship, that a man can measure himself, as Crooks points out, and, thus, find meaning in his life. As Ma says in The Grapes of Wrath, "...if all people who are shoved off the land get mad together, they can take action."