Steinbeck’s attitude toward the subject of his novella is one of great sympathy tempered by realism. Steinbeck contrasts the real world of poverty, limited resources, limiting social roles, human intolerance, and violence with the dream world of freedom, autonomy, wealth, friendship, and loyalty. This stark and unflinching juxtaposition seems an honest attempt to reveal that for some, the American Dream was simply that – a hopeless dream.
It’s not surprising that Steinbeck’s tone is so terribly dismal, given Steinbeck’s attachment to "naturalism." For naturalist writers, characters are essentially "human beasts." They are influenced by (and should be studied through) their relationships to their surroundings. The naturalist writer analyzes his characters as though they are conditioned and controlled by environment, heredity, instinct, or chance. Because of this objective outlook, the author portrays his characters impartially, without moralizing about their "good" or "evil" natures.
Lennie’s character is a great example of Steinbeck’s naturalist approach: Lennie has some kind of mental disability, and this circumstance of his life influences his actions. Though Lennie inarguably commits a terrible crime, the way Steinbeck portrays him makes us hesitant to pass judgment. Steinbeck presents the characters to us just as they are, never insisting that we think this or that about them. Instead, his approach makes us feel for them as fellow human beasts, caught in the inevitable suffering of existence.