Steinbeck's novella begins and ends in the clearing outside Soledad, a town whose name is connotative of solitude and loneliness.  Yet, it is a peaceful place where rabbits come out of the bushes to sit on the sand in Chapter One.  George wants to spend the night there because they can have a peaceful evening together before reporting to the ranch for work.  But, he complains about the bus driver's lie that they woud only have to walk a short way.  And, even here in this halcyon place, the men cannot escape conflict as Lennie exasperates George with his capture of a mouse and his demand for ketchup, clearly foreshadowing troubles to come.  In fact, George's words are prophetic as he angrily complains to Lennie,

"I wisht I could put you in a cage with about a million mice an' let you have fun."

But, the warmth of their friendship emerges as George's voice becomes "deeper," and he recites their dream for Lennie. And, peace returns to the clearing as '[T]he sycamore leaves shispered in a little night breeze."

The story opens with a lot of description.  The Salinas River is described, as well as the animals around it.  Finally, George and Lennie are introuced.  By introducing the nature first, a mood that is both somber and elated emerges.

A focus on an idyllic and completely natural world sets the stage for two things:

1. It's possible that George and Lennie really belong in the natural world. In this setting, they are free to dream and to be, generally, free. When they arrive at the ranch, the setting and emotional atmosphere is presented in an opposite fashion - closed; constrained; defeating. 

In this way, the opening chapter serves as a contrast to life on the ranch.

2. There is a larger world than the world of man. The wild and peaceful brook is the scene of a world that does not require men and does not even really acknowledge them.