What is it that Curley's wife, Crooks and Candy all have in common and what prevents them from sharing (coming together over) this commonality?
The cast of characters in Of Mice and Men is largely made up of people who are socially powerless in one way or another. This shared trait connects Crooks, who is a racial minority and also physically handicapped, to Candy, who is rather old and is missing one hand, and to Curley's wife, who is the only woman living on the ranch.
Crooks and Curley's wife are both practically isolated. Crooks lives alone in a room in the stable and Curley's wife is subjected to rules of social decorum that essentially force her to stay in the house by herself (if she were to follow the rules). Thus, these two characters are isolated by social bias.
Candy, like the other ranch hands, is relatively poor (he owns no property and must toil for a living without much hope for financial gain). His age and his financial status combine to render him socially powerless. In contrast to Slim or George, men possessing physical strength and a strength of intellect, Candy possesses only the thinnest claims to the social capital associated in the world of the novel with his gender and race.
The same forces that isolate Crooks and Curley's wife from the other people of the ranch can be identified as being responsible for isolating these figures from one another. Social bias (relating to race and gender) stipulate that certain rules must be followed. Crooks has to live apart from the other men because he is black. Curley's wife has to live apart from the men because she is a woman.
Candy's isolation from these two figures is indicative of the same codes of decorum in the society presented in the novel, yet his connections to these other two characters can be seen as speaking to a deeper point being made in the work. Social privilege and social capital are derived from sources that are often superficial. There is no moral quality that distinguishes Curley from the men - - the only quality that sets him apart is his money. There is no moral quality (or failing) that sets Crooks apart from the other men - - the only difference between them is skin color.
Indeed, isolation is a key theme in the novel. George speaks to this point early in the novel.
"Guys like us that work on the ranches, are the loneliest guys in the world. They got no family. They don't belong no place."
Interestingly, the characters in Of Mice and Men do have a place in a pejorative sense. And this is the point we are looking at here.
Crooks and Curley's wife are supposed to stay "in their place" and not challenge a status quo that favors white males (with money). They have a "place" but the identity associated with that place offers no power. Socially, these characters are marginalized in ways that are paralleled in Crooks' living arrangement and in Curley's wife's spoken recognition (when she talks to Lennie) of the expectations that keep her locked in the house alone.