To reduce the complexity of John Steinbeck to social commentary would be a mistake. As previous answers have already mentioned, Steinbeck explored numerous themes on the human condition, including friendship, desire, relationship to the land, alienation, etc... That said, since no other answers have addressed this yet, I will mention that there is indeed a strong current of social/political commentary in his work.
The Grapes of Wrath is perhaps the most obvious example. Disenfranchised farmers travel across the country. In a world moving toward more industrialized agriculture, the society doesn't appear to have any interest in supporting the people these changes displace. This leads many characters, including Tom Joad, to reconsider the values of human and social relationships. The image of the turtle struggling to cross the road in chapter 3 is just one symbolic example of Steinbeck's assertion that people must help one another rather than pursuing only their own interests. This advocates a more collectivist, and perhaps even socialistic form of society.
An additional example would be Steinbeck's treatment of marginalized groups of people in Cannery Row. Both Chinese immigrants and prostitutes face unfair criticism due to their background and line of work. While these are some of the kindest and hardest-working characters in the book, the society does not always treat them as such.
These are just two of many examples of Steinbeck's continuing focus on social justice in his novels. He had a lifelong passion for giving voice to people society treated unfairly. This is perhaps a key them of his work that I might add to what has already been written.
Theme Statements for Of Mice and Men
Friendship: In Of Mice and Men, Steinbeck shows that friendship is important in preventing loneliness by providing a purpose in life and a sense of companionship.
Loneliness: In Of Mice and Men, Steinbeck shows that leading a life of loneliness causes isolation and leads a person to build walls instead of bridges.
Outcast: In Of Mice and Men, Steinbeck shows that being an outcast can cause a person to build walls and leads to a life of loneliness.
Dreams: In Of Mice and Men, Steinbeck shows that having a dream, which is possibly out of reach, can make life bearable.
In Of Mice and Men, Steinbeck shows that having a dream, which is possibly out of reach, can hurt the dreamer.
Theme Statements for The Pearl
Belonging: In the Pearl, Steinbeck argues that, although man occupies a small space within the larger universe, the desire for roots and a place to call home gives individuals a sense of belonging.
Theme Statements for East of Eden
Happiness: Steinbeck questions if material goods can bring happiness. He argues that true happiness comes from meaningful relationships between individuals.
Other Themes in Steinbeck's Novels
- The American Dream
- Mis-Treatment of Women
John Steinbeck was a fairly focused writer in terms of the themes he explored.
In his books To a God Unknown and East of Eden Steinbeck takes up the themes of identity through land ownership as part of the American vision of fulfilling a destiny. In both these works, there are sub-themes dealing with the indifference of nature to man and man's attempts to innovate as a compromise with nature. Each of these novels also presents a character who can be said to express a thematic idea of happiness-by-denial, where a man insists upon maintaining a positive outlook despite many depressing circumstances. Some of these themes can also be found in The Pearl.
In Tortilla Flat, Cannery Row and Of Mice and Men, Steinbeck explores various themes focused on friendship and the moral compromises friendship can create.
Political themes of disenfranchisement, rebellion, and integrity vs. corruption are explored in Of Mice and Men, The Grapes of Wrath, The Pearl and The Moon Is Down.