How does Steinbeck present the feeling of loneliness in Of Mice and Men?

In this novel, the workers themselves lead lonely lives. Although most of the story shows the majority of the men working and living together, these itinerant ranch workers often changed jobs frequently and moved on to other jobs individually. George, and others in the story, note how this leads most men to waste all of their money on alcohol and prostitutes. With no family, no settled home, and no continuous friendships, they resort to this lifestyle. 

George and Lennie endeavor to break this trend. By sticking together, they will avoid that lonely fate and instead of wasting their money, they are attempting to save it and buy their own land. It is a dream that keeps both of them going, but it is particularly encouraging for Lennie. George always recites for Lennie how most workers waste their lives, whereas he and Lennie are pursuing a dream together: 

Guys like us, that work on ranches, are the loneliest guys in the world. They got no fambly. They don’t belong no place. They come to a ranch an’ work up a stake and then they go into town and blow their stake, and the first thing you know they’re poundin’ their tail on some other ranch. They ain’t got nothing to look ahead to. 

With us it ain’t like that. We got a future. We got somebody to talk to that gives a damn about us. (Chapter 1)

Were it not for George, Lennie would be completely alone. With no social skills, he would be a social outcast. 

The other notable lonely characters are Curley's wife, Candy, and Crooks. Curley's wife is the only woman on the ranch. She is constantly seeking attention and she laments the fact that she missed her chance to pursue her own dream: 

Coulda been in the movies, an’ had nice clothes—all them nice clothes like they wear. An’ I coulda sat in them big hotels, an’ had pitchers took of me. (Chapter 5) 

Candy feels encroaching loneliness. In other words, in his old age, he knows that he soon will not be of much use as a worker. With that fear lurking, he latches on to George's and Lennie's dream of owning a farm and offers to work with them. 

Crooks is perhaps the loneliest character in the novel. Although he works "with" the other ranchers, he is relegated to his own living and sleeping quarters. The other men do not invite him when they go out for the evening. Naturally, Crooks is standoffish when Lennie tries to befriend him in Crooks' own room. He suggests that if they don't accept him, why should he accept them. However, he warms up to Lennie and begins to share a bit of Lennie's dream of owning his own land. That interest is squashed when Curley's wife shows up and threatens him. With her position as the boss's son's wife, Crooks retreats.