Of Mice and Men is a cyclical story, because the setting is the same at the end and the beginning.
The setting of the story is important because Steinbeck takes great pains to describe the beautiful and peaceful nature scene of the Salinas Valley. Before we meet any characters, we meet the 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. (chapter 1)
This pool is symbolic of the cycle of life. In the beginning of the story, the first thing Lennie does is sink his head into it to drink. George scolds him, telling him not to drink so much. George is taking care of Lennie, through direct interaction with the setting. They discuss their dream of having their own land someday, where Lennie can tend to the rabbits.
At the end of the story, the two men return to the same pool. George knows he has to end Lennie’s life, and he distracts him by reminding him about the rabbits just as he did in chapter 1.
Lennie turned his head and looked off across the pool and up the darkening slopes of the Gabilans. “We gonna get a little place,” George began. He reached in his side pocket and brought out Carlson’s Luger; he snapped off the safety, and the hand and gun lay on the ground behind Lennie’s back. (chapter 6)
George is still taking care of Lennie, by shooting him. He knows that Lennie does not understand what he did wrong, but he also knows Lennie is a danger to others.