One of the major themes of Steinbeck's novella Of Mice and Men is the importance of friendship, especially for the working men on the ranch who have mostly been marginalized and isolated. At one point Slim points out that it's funny to see two men travel together as George and Lennie are doing. He asserts, in chapter three, that the laborers usually travel alone and don't care about each other:
"Hardly none of the guys ever travel together. I hardly never seen two guys travel together. You know how the hands are, they just come in and get their bunk and work a month, and then they quit and go out alone. Never seem to give a damn about nobody."
Fortunately for George and Lennie, they have each other and their friendship appears to be quite strong. George admits that Lennie is a "nuisance" but he continues to travel with him, and they form an important bond. Likewise, Candy sees the old dog that he raised from a puppy as his close friend, and, as with George and Lennie, they have a meaningful bond. It is why he balks at the thought of killing the old dog, despite the dog's infirmities. Candy's offer to join George and Lennie in their quest for their own piece of land is compensation for the loss of his only friend.
Even before the old dog is taken out and shot by Carlson, Steinbeck weaves in his ideas about friendship when he brings up the letter in the pulp magazine which Whit shows to Slim. What is seemingly a meaningless scene in the book is really a commentary on how important friendship is for these working men. Whit had worked with the man, Bill Tenner, who wrote the letter, and they had become friends. Whit says, "Bill was a hell of a nice fella." This display of affection is juxtaposed with Carlson's selfish demands that Candy's dog must be put out of its misery. Carlson's lack of empathy for friendship is again shown in the book's final scene when he fails to understand George's grief over killing Lennie. He remarks ignorantly as George and Slim walk away from him, "Now what the hell ya suppose is eatin' them two guys?"