Certainly, the friendship of Lennie and George underscores the major theme of friendship. Perhaps, then, it would be better to state the broader idea of Friendship, or Fraternity of Men, as a theme. By so doing, therefore, the writer can include other characters with George and Lennie, such as Candy and even Crooks for a time.
With this fraternity of the "bindle stiffs" of Steinbeck's classic novella in mind, the theme of Fraternity is illustrated in the ways in which Lennie is devoted to George, George acts responsibly toward and on behalf of Lennie, Candy is willing to share his money with the others in order to better all of their lives, and Crooks offers to work hard on the dream ranch if the others will allow him to live with them.
Once these men are joined in their thinking of owning a share of a place of their own, and they are happier as their lives have been given meaning since meaning does depend upon sharing. As Crooks tells Lennie,
It's just the talkin'. It's just being with another guy. That's all."
In addition, there is a hope for the future as George states,
With us it ain’t like that. We got a future. We got somebody to talk to that gives a damn about us.
Tragically, after Lennie kills Curley's wife, all the hopes of Curley, Candy, and Crooks are dissolved, for it is only in their hope of joining together, or fraternity, that they can be safe from the alienation and destitution of their present situations.