In John Steinbeck's Of Mice and Men, Lennie is the keeper of the dream. As the author himself has written, "Lennie was not to represent insanity at all but the inarticulate and powerful yearning of all men." This yearning is the American Dream of owning one's own property, of having some financial independence and of belonging somewhere.
The small American Dream that George and Lennie possess is to own some land that they can farm. On this property Lennie can raise rabbits and have them as pets. There, he and George hope to grow their own food and be self-sufficient where they will live in, not fear, but fraternity--"because I got you to look after me, and you got me to look after you, and that’s why.”
Although theirs is a modest aspiration, the dream of George and Lennie is impractical during the Great Depression when the men made little more than what kept them alive. As "bindle stiffs," they reside no where, instead traveling wherever work takes them. But, because Lennie is mentally a child, he holds onto this optimistic dream, insisting that George repeat its mantra until George himself almost believes in it. And, herein lies Lennie's importance as a character. With childish optimism, Lennie gives George hope; then, he inspires Candy, and then Crooks. He even makes Curley's wife believe that something positive my happen to her. Slim perceives the positive effect that Lennie has upon George and acknowledges the friendship of the two men.
Not susprisingly, the child-like candor and innocence of Lennie evokes the optimism of the men during a most desperate era. So, of course, when Lennie dies, the tone of the narrative changes abruptly. Old Candy despairs; Crooks is forsaken, George knows there is no way to retrieve the past; the dream dies. Bereft of his friend with nowhere to go in the Great Depression, George walks up the hill in abject despair.