What was steinbeck trying to say by writing this novel?

Steinbeck was criticizing American society to an extent. I think the themes of the novel shed more light on his reasons for writing it. Human nature, loneliness, isolation and loyalty are all mixed together on a canvas, so to speak, giving us tapestry of these itinerant workers during the depression. Steinbeck looks into the hearts and minds of people forced to live a life of loneliness and isolation. Each case is unique yet the same. It's a paradox that can be seen in every character from Curly's wife to the black stableman Crooks to the mentally challenged Lennie.