How and why does Steinbeck change the mood of the opening scene in Of Mice and Men?

John Steinbeck's use of dreams, the unconscious,
reccurring myths, and symbolic characters are characteristic of what Carl Jung, the
founder of analytical psychology, called the "visionary" style.  Steinbeck himself once
noted that realism is the surface form for his interest in psychology and philosophy. 
His opening scene in Of Mice and Men illustrates just this realism
that suggests dreams, the unconscious, the evolutionary and the
visionary.

Fed by the Salinas River, the Salinas Valley is
lush and fecund, and is known as "The Salad Bowl" of the world because of its lettuce
and many vegetables. Yet, located in the central Salinas Valley is the town of Soledad,
which means solitude or loneliness. These contrasting settings
suggest Steinbeck's call of a lost Eden to the alienated itinerant workers who enter the
scene.  There seems, too, an almost evolutionary stage to Lennie, who is described in
animalistic terms.  For instance, after dragging his feet "as a bear drags his paws,"
when he comes to the river, Lennie places his face in the water and "dabbled his big paw
in the water."  In contrast to Lennie, George stares morosely at the water, and, perhaps
sensing the conflicts to come as he and Lennie will again experience a change in their
lives, George complains of having to have Lennie
around:

"I
could get around so easy and so nice if I didn't have you on my
tail." 

Right before the "day
was going fast now," with a little wind that dies down, George discovers a dead mouse in
Lennie's pocket and throws it across the pool into some brush.  But, his anger fades as
he realizes that Lennie does not understand and he remembers his obligation to Lennie. 
Then, acquiescing to the myth, George recites the dream of owning a ranch with rabbits,
like the rabbits that have taken cover from the men.

In
this visionary opening scene, very craftily Steinbeck has suggested the events of his
entire narrative.  The lost Eden, the lost dream; Soledad not far away, the men's
loneliness; the dead mouse, the dead woman; the rabbits running for cover, the flight of
Lennie; "the day going fast," the end of their short employment and their friendship. 
Because all these events are foreshadowed, the opening scene is one of changing moods
from peaceful hiatus to conflict and to dream, then to
nightfall.