The theme of isolation is very prominent in the story. We see this in nearly every character.
Certainly the most isolated character in the story is Crooks. He is the lone black man on the ranch, and he has no community. For example, he does not live with the men in the bunk house. He lives alone, and no one visits him. Crooks admits this, and he says that he can go crazy, because he has no community.
"Books ain’t no good. A guy needs somebody—to be near him.” He whined, “A guy goes nuts if he ain’t got nobody."
Candy and Slim are not much better. To be sure that they have the other men in the bunk, but for the most part there is no real friendship. Slim makes this point in the beginning of the novel.
Slim looked through George and beyond him. “Ain’t many guys travel around together,” he mused. “I don’t know why. Maybe ever’body in the whole damn world is scared of each other.”
What sets George apart and puts him in a better place is that he has Lennie, and Lennie has him. In short, they got each other. This is one of the refrains in the book.
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.
Lennie broke in. “But not us! An’ why? Because . . . . because I got you to look after me, and you got me to look after you, and that’s why.”
From this perspective, the theme of isolation is built into every character except George and Lennie. That said, at the end of the book, when George kills Lennie, isolation wins out.