George is correct. If one theme can be thought of as defining the plot and symbolism of Of Mice and Men, that theme is loneliness. In many ways, from the outspoken to the subtle (such as Steinbeck's decision to set the novel near Soledad, California, a town name that means "solitude" in Spanish), the presence of loneliness defines the actions of the diverse characters in the book.
The itinerant farm worker of the Great Depression found it nearly impossible to establish a fixed home. These men were forced to wander from ranch to ranch seeking temporary employment, to live in bunk houses with strangers, and to suffer the abuses of arbitrary bosses. George sums up the misery of this situation at several points during his monologues to Lennie - "Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place" (15).