I believe that Steinbeck is saying that men must do what men must do. Men must take work that comes to them. Men must be able to "put up or shut up". Men must be able to work physical jobs that require physical attributes that only men have.
Think about Crooks; he is not really considered masculine by the other ranchers. First, he is black and, therefore, the other ranchers (white) do not look at him as a true part of the team. Second, he is crippled. He can no longer perform in such a way that the other ranchers can. Similarly, Candy is not seen as a man on the ranch either. Since the loss of his hand, Candy feels like he is only kept on as a hired hand because the owner feels sorry for his accident, which happened on the ranch.
Curley, fearful of Lennie because of his size, does not qualify as a true man either. Given his loss to Lennie during their fight, many of the ranchers feel as though someone finally put Curley in his place.
Slim and George seem to be the only true men in the novel. Slim is looked up to by all of the other ranchers and George does what he has promised to do in life-take care of Lennie. Both men live up to the standard of being a true man.
I will take a detour and focus on Curley. He was completely intimidated by Lennie's size and immediately wanted to start some trouble with him. Moreover, he consistently channeled his frustration regarding Lennie by bullying him.
This is what keeps making me wonder, though. Curley is fixated with Lennie. When his wife dies, he makes a minimal attempt at grief and his first reaction is to go after Lennie.
This being said, I feel that Steinbeck was taking a shot at males who cannot build manhood upon self sacrifice and the building of character. He was clearly attacking those "heirs" of farms and other businesses that could only be there by association, and not by right.
Keeping in mind that John Steinbeck wrote in the style of Naturalism, the reader may perceive that the "bindle stiffs" separated from their families and loved ones are much like the sole animal that must fend for itself. Therefore, these men become cruel in their fear, as George mentions when he says,
"I seen the guys that go around on the ranches alone....After a long time they get mean. They get wantin' to fight all the time."
With the Great Depression as the setting of his novella, Steinbeck's men are reduced to the Darwinian level of the struggle for survival. If they show physical prowess, therefore, they will fend off rivals for work or for their women. That most of the men's names begin with the same letter--C--is no coincidence. Steinbeck suggests that they are all the same in their yearnings and struggles, having been reduced to basic survival level. The resulting alienation of this struggle is evidenced more strongly in the further compromised social outcast, Crooks, and the mental outcast, Lennie, who, in Steinbeck's own words,
... was not to represent insanity at all but the inarticulate and powerful yearning of all men."
This yearning is the natural one of survival, first of all. After that, there is the desire for fraternity, with which men can fight the terrible aloneness that haunts the soul. And, as a Socialist, Steinbeck strongly believed in the unification of men to bring about effective social change.
I think that Steinbeck is basically setting up George against Curley as competing visions of what a man should be.
Curley is macho. The fact that he's a boxer and the fact that he treats his wife the way he does shows this. His glove is a sign of his aggressive sexuality. This is one image of what a man is -- aggressive and domineering.
George has moments where he is like that, where he bullies Lennie. But overall, he presents us with a vision of masculinity where a man is a person who takes care of those who need him. This is a much less macho, less aggressive vision of masculinity.
In John Steinbeck's Of Mice and Men, a great deal of what Steinbeck has to say is from a first-hand experience growing up in Salinas, California, where he once worked on a ranch like George and Lennie. He saw the conditions under which migrant workers lived…except the migrant workers in Steinbeck's novels consisted of the disenfranchised who lost everything in the Great Depression. George and Lennie are independent enough to move from place to place more easily than migrant workers with a family to support—thousands moved across the U.S. to find work. (Steinbeck writes about this in The Grapes of Wrath.)
Lennie and George are extreme opposites. George is hard-working, protective of Lennie, and bright—perhaps cynical: life for them is hard. George knows what they are up against and what he needs to be prepared for, especially in keeping Lennie out of trouble. George is also lonely because he is always on the move—sometimes against his will—caring for another person.
Lennie, with his mental disability, is very strong, but has little understanding of the world, or his strength; he forgets some things, but is able to recall clearly the story of the house they hope to have one day: the American Dream. He is a gentle man at heart, but strong physically—without a concept of self-control.
Curley is a nasty, insecure man that George and Lennie work for at the ranch. He has power, but his insecurity about his own masculinity and doubts regarding his wife's faithfulness make him a bully. He tries to harass Lennie: a big mistake—Lennie easily crushes his hand.
Candy is another man who represents those who have no control over their lives: he cannot stop the shooting of his old dog. Because of his own age, he knows that he will not be allowed to stay on when he can no longer keep up with his work. Crooks is also a ranch hand, but he is black. He lives in an old shack attached to the barn because he is not allowed to bunk with the white men. He also wants a part of the American Dream, but he, too, has no control—but it's because he is black. Curley's wife threatens to have him lynched, and he crumbles—which shows just how powerless he is.
Curley's wife has dreams of being a film star, hates being isolated on the ranch, and stirs things up because she will not stay away from the men; she comes around with flimsy excuses to strike up conversation. She is lonely, too, but does not need to worry about her future. Her marriage affords her a great deal more confidence than the men on the ranch have. It is ironic that this woman has more power than the men.
In the heart of the Depression, Steinbeck sees these men as creatures without control over their destiny. They are all—much like Crooks—enslaved to those who "have." One word from Curley and any one of them could lose his job. These hard-working Americans have been emasculated by the economic hardships of the time; the sense of self-doubt, and financial insecurity force them into submissive behaviors that no man, regardless of color, should have to live with, and they are distrustful—the startling speed with which America's economy toppled took the world by surprise: these men cannot count on the future.
Steinbeck's novel demonstrates how loss of control over one's life can demoralize anyone, but especially men— they have been raised to believe they can and should be able to make a living, supporting self and family. It's a part of who they are, but this also has become a dream.