In Of Mice and Men, Steinbeck uses Lennie's and George's relationship to convey ideas about America in the 1930s. George and Lennie were migrant agricultural workers on a California ranch. Lennie and George have to struggle to survive. They share a dream of owning their own farm someday. The dream of owning their own farm and farmhouse keeps them going in the hard times of the Great Depression. With the Great Depression of the 1930s, America was facing difficulties. Wages were low. Combines were replacing men. George and Lennie were fortunate when they found work. George and Lennie dream of "how they are someday going to get out of the lonely life of itinerant farm laborers and buy a piece of land where they can live by working their own small farm together."

Truly, life was lonely for the migrant agricultural workers. Fortunately, George and Lennie had each other to keep one another company. However, with their companionship, there were problems. Lennie was mentally challenged, and he kept George frustrated much of the time.

Lennie was a big man with the mentality of a child. Lennie in his child-like innocence never meant to harm anyone. Because of Lennie's tendency to get in trouble, George and Lennie were often on the run. They only dreamed of settling down one day and farming their own farm.

Of Mice and Men tells the story of two simple men who try to escape homelessness, economic poverty, and emotional and psychological corruption. Otherwise, the fate of those who do not abandon the lives they lead as itinerant workers is bleak and dehumanizing.

By the end of the story, Lennie has caused major problems. He accidentally kills Curley's wife. Because George would rather see Lennie dead than to hang at the hands of Curley, George shoots Lennie in the back of the head. Ironically, George is sharing his dream of having his own farm with Lennie as he is about to shoot Lennie. Lennie dies dreaming of a better life. In the 1930s, dreaming was all that a man had. Unfortunately, some dreams never came true. George gave up on his dream when he had to shoot his best friend Lennie. Life continued to be a lonely existence for George. 

In John Steinbeck's novella Of Mice and Men, how is George and Lennie's relationship used to build a major theme in the novel?

The major theme in John Steinbeck's Of Mice and Men, as it was in most of his early novels and stories, is compassion for the hard lives of the working poor. Many of his characters show courage and strength in spite of the hardships they have to endure. The relationship between George and Lennie is a good example of this theme of compassion for the working poor. George not only has to struggle for his own survival, but he is burdened with a dependent who is constantly causing him annoyances, frustrations, and serious troubles.

When the story opens these two bindle stiffs have been fleeing from a lynch mob in Weed, which is located hundreds of miles to the north, and they are on their way to another back-breaking job in the hot sun for starvation wages. George shows the strength of his character by not abandoning Lennie and breaking his promise to Lennie's Aunt Clara. He complains to his friend:

"God a'mighty, if I was alone i could live so easy. I could go get a job an' work, an' no trouble. No mess at all, and when the end of the month come I could take my fifty bucks and go into town and get whatever I want. Why, I could stay in a cat house all night. I could eat any place I want,  hotel or any place, and order any damn thing I could think of. An' I could do all that every damn month. Get a gallon of whisky, or set in a poolroom and play cards or shoot pool."

Yet when Lennie volunteers to go off on his own:

George said, "I want you to stay with me, Lennie. Jesus Christ, somebody'd shoot you for a coyote if you was by yourself. No, you stay with me. Your Aunt Clara wouldn't like you running off by yourself, even if she is dead."

The other men have their misfortunes and survival problems too. Candy and Crooks are the most pathetic examples. There is no one to give them any help or sympathy. If they can't work, there are plenty of others to take their places.

Steinbeck does not suggest a solution to the situation he dramatizes. His intention is to reveal it and leave it to others to propose solutions. The reader is left sharing Steinbeck's compassion for the humble human beings he writes about, and this was undoubtedly his purpose in creating Of Mice and Men.