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.