What is significant about the setting in Of Mice and Men by John Steinbeck and what does the novel tell us about friendship?

The setting does change a little in Steinbeck's Of Mice and Men.  The story begins and ends at somewhat of an idyllic spot on the edge of water where George and Lennie first appear, and ends in the same spot, where George shoots Lennie. 

I say somewhat idyllic because the place is not ideal, it's just the best that Lennie and George get in the novel.  The water is stagnant, and possibly unsafe to drink.  And the spot includes at least one dead mouse, the mouse that Lennie has in his pocket when they first arrive at the site, and that George makes Lennie throw out, because its dead and presumably decrepit.  The mouse, of course, is indicative and possibly symbolic of other dead things in the work, including the pet puppy, Candy's dog, Curley's wife, and Lennie himself. 

The setting does reflect George and Lennie's dreams, of course, including even the rabbits. 

Notice again, though, that the rabbits vanish before George and Lennie arrive--Lennie doesn't get to see them. 

He never gets to see them on a place of his own, either.  This setting is reflective of the rest of the novel and Lennie's fate.