John Steinbeck's novella, Of Mice and Men, has many different themes highlighted within the text. That said, one theme (set in contrast with another) is the idea of friendship and loneliness. While it may seem that each are far too contrasting to be aligned, the ideas presented with each tie together through the contrast.
Essentially, the idea of loneliness is paired with the idea of friendship based upon the relationships depicted within the novella. While the only true friendship seen is the one between Lennie and George, both are essentially lonely. George is lonely because he is contrastingly different from Lennie. While the friendship exists, it is only based upon necessity (a promise made to Lennie's aunt). That said, Lennie is also lonely in the text. His search for friendship is constant (he is always seeking out things to bring him companionship).
As for the other men in the novella, none of them have a true relationship with the others. While each may look up to another, the men on the ranch are all about the individual. For example, Carlson wants Candy's dog put down because it annoys him. If he were really friends with Candy, he would not push for the dog's death. Instead, he would embrace his friend and understand Candy's need for his dog.
Essentially, the theme of loneliness exists as a contrast to friendship. By highlighting one, readers are able to see the highlighting of the other. Therefore, one can readily identify friendship given they can also identify loneliness. Each need to be a part of the novella in order for the other to be apparent.
"Guys like us, that work on ranches, are the loneliest guys in the world...With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us."
The contrast must be set up for the novella to progress as it does, for the conclusion to make sense, and for the relationship between George and Lennie to be understood by the readers and the ranchers alike.