The suggestion by post#2 is perfect for Steinbeck's novel; as a socialist, Steinbeck was so concerned about underdogs. Within the setting of this novel, the "bindle stiffs" were thousands of white men who were displaced after the Dust Bowl and other economic losses of the Great Depression. Because they were itinerant they were unable to organize as others did in California so that they were not be oppressed and be paid fair wages. In other novellas and novels, Steinbeck champions other races, but they, like the "bindle stiffs," are poor and, therefore, oppressed by upper classes.
Another thesis could involve the importance of dreams. The American Dream has been a great motivator for many. Lennie and George's dream keeps them going while at the same time, it keeps them as friends.