The main way Steinbeck portrays friendship is through the relationship between Lennie and George. Lennie is mentally handicapped and relies on George to guide and take care of him as they travel from job to job as migrant workers. Yet they are united by a true bond of friendship and care deeply about one another.
This friendship differentiates them from most of the other men they meet as ranch workers. As George says to Lennie, migrant workers are the loneliest of people. But he and Lennie have each other. George states:
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.
As they enter the new ranch, they meet with men who are lonely and isolated because of their work and inability to put down roots. The other men express a great longing to be part of George and Lennie's dream of owning a small farm, because then they too could have a sense of community and belonging.
One of the tragedies of George having to kill Lennie is how the loss of his friend will impact him: he will be as lonely and hopeless about the future as the other men he meets on the road.
While George and Lennie are extremely lucky to have the close friendship they do as long as they do, Steinbeck's larger point is that one cruelty of capitalism is the way the endless tramp from job to job and an uncertain future robs people of commonplace joys of friendship and a stable home life.