Loneliness is a major theme in Of Mice and Men. George is talking with Lennie about loneliness in chapter one:
Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don’t belong no place. . . . 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 blowin’ 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 gives a damn. But not us.
George is talking about how fortunate he and Lennie are to have each other. It is a lonely life for guys like George and Lennie. They have no other family members. They move from ranch to ranch. They meet other ranch hands who are very lonely.
Crooks is lonely. He is isolated from everyone else because he is black:
Crooks, the despairing old Negro stable worker, lives alone in the harness room, ostracized from the ranch hands.
On the one occasion when Crooks talks with Candy and Lennie, he is negative and tells them they will never attain their dream of owning their own place:
Crooks tells them they will never attain their dream. Crooks is excluded from the rest of the ranch hands.
Candy is lonely. He invites himself to be a part of George's and Lennie's dream of becoming a home owner. He feels lesser than the other ranch hands because of his injury. He finds much joy in dreaming right along with George and Lennie about one day buying a home:
Candy is the old, disabled ranch hand who is helpless to stop the shooting of his dog and who knows that he too will be banished when he is no longer useful. He is sweetly hopeful of joining Lennie and George on their dream farm, offering to contribute his savings of $350 to buy the farm.
No doubt, the novel is filled with lonely characters. Curley's wife is perhaps the loneliest of them all. She has no other female companions. She is surrounded by men. She reaches out to someone like Lennie due to her loneliness:
But she is pathetically lonely and had once had dreams of being a movie star. Both she and Crooks crave company and "someone to talk to."
Lennie is lonely too. He reaches out to Curley's wife. He accidentally breaks her neck and George realizes the dream is over:
On Sunday afternoon, while the others are playing horseshoes, Curley's wife gets Lennie to feel her soft hair. When he begins to muss it, she panics, and he accidentally breaks her neck. When George discovers what has happened, he realizes that their dream is over.
Truly, the characters in Of Mice and Men live a lonely existence. Even having each other does not fill the void at times.