The loneliness and isolation experienced by the migrant workers depicted in the text are certainly central to the themes of Steinbeck's Of Mice and Men.
Isolation based on gender, race and class can be described as being a salient element of the microcosm of American society presented in Of Mice and Men, and loneliness is the inevitable result.
For examples of isolation, we can look at Crooks and Curley's wife in particular as figures isolated by categorical, demographic characteristics. Crooks is black and so is set apart, literally, from the rest of the people on the ranch. Curley's wife is denied the opportunity to develop any friendships due to her gender (and certain practical fears derived from her sexuality and attitude).
Class is another significant divisive force in the text as the ranch hands like George and Lennie are not only separated from the ownership of the ranch in a bunk house but are also separated from the other ranch hands by an ethos of mutual suspicion and self-interest. The men struggle to save any money from their wages and live in constant competition with one another. Their state of relative poverty ensures that they will remain in this state of competition, working against one another to get jobs instead of working together to build a different way of life.
That different way of life is articulated in the novel in the shared dream of land ownership discussed by Lennie and George (and then Candy and Crooks too). The men have a vision of working together in a system of partnership and cooperation. In this vision they will also retain the value of their labor, building capital for themselves—or at least being in control of their own schedules.
Thus the shared dream of land ownership highlights the current state of isolation that the men endure.
The friendship between George and Lennie functions similarly in the text, drawing a contrast between this special circumstance and the more normal situation experienced by men of their station.
George went on. “With us it ain’t like that. We got a future. We got somebody to talk to that gives a damn about us. We don’t have to sit in no bar room blown’ in our jack jus’ because we got no place else to go. If them other guys gets in jail they can rot for all anybody give a damn. But not us.”
The emphasis here is on how friendship is outside of the norm. The bond shared by George and Lennie is far from standard and instead serves to illuminate the isolation that is so common among migrant workers in this era of American life.