just to survive.

Steinbeck wants to illustrate that the loneliness is not something that hits a few random individuals but is widespread. The best way to portray that is to show how so many of the ranch hands react favorably to the dream of being part of the farm Lennie and George are saving to buy. They know they would be far less lonely if they could put down roots in one place and own a small piece of land. Ironically, this was once the American Dream, but Steinbeck shows it as out of reach for these men during the Great Depression.

You could look at how Candy and Crooks, marginal, vulnerable characters, react to the idea of a farm. You could also look at how Steinbeck explains loneliness as a destructive cycle that causes the men to blow their wages on drink and prostitutes rather than saving and pooling their resources to build a better future.

In trying to understand your question, it seems you have already answered it. Therefore, I will help you support it.

Before the story, George hooked up with Lennie somehow. Most people don't go out of their way to choose a disabled person to befriend, however George seems to be rather welcoming of Lennie's companionship even if it is a pain most of the time. This was desperate.

During the story, we see several examples of lonliness leading to desperation. Curley's wife says she could have been in pictures. She seems to have struggled with her mom because mom hindered that life for her. I think running away and marrying Curley of all men (and it sounds like this happened on a whim) was a desperate act borne of the lonliness she felt being cooped up by her mother.

Crooks also took a shot at Lennie when Lennie came to visit with him as a result of lonliness. If Crooks had any practice at being friends, he would not have uttered the hurtful words he did to Lennie in suggesting that George would desert him.

There are two possible thesis statements below that focus on the loneliness and friendship themes in Of Mice and Men.

In Of Mice and Men by Steinbeck, the writer shares the loneliness that ranchers faced in the time of The Great Depression by expressingthe fact that George, who is a cranky, cynical old man, decides to befriend a mentally challenged Lennie and accepts the burden of protecting Lennie from harm, even to the point of George having to kill his best friend to protect Lennie from Curly's torture. 

While loneliness is definitely a theme in Of Mice and Men, Steinbeck creates a central idea relating to the friendship between George and Lennie that is so real and unique until the reader is convinced that George loves Lennie more than himself, for at the end of the novel, George has to shoot and kill Lennie to protect him from Curly's torture, ultimately knowing he himself--George--will suffer from the loneliness of not having Lennie with him to keep his dream of owning a ranch alive.  

The writer uses dialogue below as George expresses his loneliness and his friendship with Lennie in their conversation as they sit by the water. This is an excellent quote in support of the thesis statements above.

"Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place. . . . 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" (15).

Of Mice and Men (a novelette) was written by John Steinbeck, set during the Great Depression in the United States.

Steinbeck uses the theme of loneliness not only in "Of Mice and Men," but also in The Grapes of Wrath, also set during the Great Depression.

Because of the devastated economic system after the Crash of 1929, many people lost jobs, had to sell almost everything they owned, and often had to move to find work. This was especially true of those who lived on farms or outside the cities, where jobs had dried up and the people were hit especially hard.

In "Of Mice and Men," George and Lennie must travel to find work. What makes it even more difficult is Lennie's mental handicap and his inordinate strength: coupled together, these characteristics cause problems wherever they go, necessitating their rapid departure to avoid unpleasantness—even the law.

George has promised to take care of Lennie, but his life is a lonely one. More than being responsible for himself and struggling as so many other Americans were at the time, George also cares for Lennie, and it is a job that does not end until the conclusion of the story.

With this in mind, I find it paradoxical that George's only companion is his loneliness. Lennie depends on George and cares for him, but is not capable of being the friend that would ease George's sense of isolation.

I'm not sure what the basis of your essay is, however, if you are writing about loneliness, based upon what we see in Of Mice and Men, I would submit the following as a thesis statement.

Oftentimes dire circumstances are not the only things to crush the human spirit: when loneliness is a person's constant companion, it is difficult to rise above his lot in life and feel connected to the world around him.