The Scottish poet Robert Burns once wrote, "The best laid schemes o' mice an' men gang aft aglay." He wrote that no matter how much plans are carefully laid, things can go wrong. This statement is at the root of the novel. It foreshadows the consequences of two companions' dreams. They both have a wish to fulfill to change their living. However, the sad thing is that no matter how hard they try to fulfill their dreams, the plans always somehow fail. Indeed, this story gives the reader the harsh reality of life.
Set in the Depression, the two main characters, George and Lennie, are poor migrant workers on the path to wealth. Because each one has the other as a companion, both thinks that they are different from everybody else and that there's a future ahead of them. They hope to have a house of their own with a couple of acres of land.
However, Lennie, who is mentally retarded, has to be looked after by George. They're having a hard time keeping their jobs, as Lennie always does something stupid to get them in trouble. As a metaphor, Steinbeck makes Lennie as the obstacle to success.
This raises a key issue: their companionship. Can George and Lennie be considered friends? Lennie would most likely agree. George, on the other hand, probably does not see Lennie as a friend. Living with Lennie is a great danger to both of him. This was portrayed in the first chapter when Lennie touched a girl's dress. They were being hunted for rape. George confides to Slim that he has gotten used to Lennie and that he "can't get rid of him." Despite Lennie's annoyance and danger, George still keeps looking after him. His motivation probably stems from pity.