What are the important themes in Of Mice and Men?

In my opinion there are three major themes in Steinbeck's novella Of Mice and Men, including the importance of friendship, the pain of loneliness, and the idea of the American Dream.

The theme of friendship is exemplified by the relationship between George and Lennie. They travel together throughout California as migrant farm workers. They've been companions ever since Lennie's Aunt Clara died. George is somewhat of a caretaker for Lennie, who is mentally challenged....

In my opinion there are three major themes in Steinbeck's novella Of Mice and Men, including the importance of friendship, the pain of loneliness, and the idea of the American Dream.

The theme of friendship is exemplified by the relationship between George and Lennie. They travel together throughout California as migrant farm workers. They've been companions ever since Lennie's Aunt Clara died. George is somewhat of a caretaker for Lennie, who is mentally challenged. The reader may also assume that, even though Lennie is often a problem, George cares for him very much. In chapter one, George describes the friendship between the two men:

“With us it ain’t like that. 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. If them other guys gets in jail they can rot for all anybody gives a damn. But not us.” 

George ultimately shows his true friendship to Lennie by killing him in a merciful act at the end of the novel. 

The pain of loneliness pervades the novel. The reader may focus on Curley's wife and Crooks as the two loneliest characters. Curley's wife seeks companionship throughout the novel and, unfortunately, is perceived as a tramp or a tart because she often tries to talk to the men on the ranch. Her husband, Curley, is both belligerent and insensitive. The two are never pictured together in any scene until the end when the girl is dead. She talks about her loneliness in chapter five when speaking to Crooks, Candy, and Lennie. She says,

“—Sat’iday night. Ever’body out doin’ som’pin’. Ever’body! An’ what am I doin’? Standin’ here talkin’ to a bunch of bindle stiffs—a nigger an’ a dum-dum and a lousy ol’ sheep—an’ likin’ it because they ain’t nobody else.” 

In the end, Curley's wife's loneliness and search for companionship causes her death, as she is accidentally killed by Lennie.

Because he is black, Crooks is segregated from the rest of the men and tells Lennie it is hard to live alone in his room in the barn. He wishes he had someone to talk to. Crooks says,

“A guy goes nuts if he ain’t got nobody. Don’t make no difference who the guy is, long’s he’s with you. I tell ya,” he cried, “I tell ya a guy gets too lonely an’ he gets sick.” 

The American Dream is also relevant in the story. Americans have always strived for freedom, and having one's own house or piece of land is an extension of that. For George and Lennie, the dream of a "little piece of land" is something they always talk about. In chapter three, George describes the dream in some detail:

“Sure, we’d have a little house an’ a room to ourself. Little fat iron stove, an’ in the winter we’d keep a fire goin’ in it. It ain’t enough land so we’d have to work too hard. Maybe six, seven hours a day. We wouldn’t have to buck no barley eleven hours a day. An’ when we put in a crop, why, we’d be there to take the crop up. We’d know what come of our planting.” 

Having overheard George's plans, Candy offers to contribute money and become a part of the dream. For a time, with the money contributed by Candy, the three men have the dream almost within their grasp. When Lennie kills Curley's wife, George realizes the dream is dead.