One of the themes of the story is “idealism vs. reality” (enotes themes), or the contrast between the supposed peace of nature and that of man.
Steinbeck opens the book with a very peaceful setting.
A few miles south of Soledad, the Salinas River drops in close to the hillside bank and runs deep and green. The water is warm too, for it has slipped twinkling over the yellow sands in the sunlight before reaching the narrow pool. (ch 1)
The words used to describe the setting are peaceful and make you feel warm and fuzzy. This is also true of the story George tells Lennie about how they will buy a farm and tend rabbits. The reality is much different.