The American Dream is written into the Declaration of Independence: "life, liberty, and the pursuit of happiness." Lennie and George's dream of owning a farm and living off the "fatta the lan" symbolizes this dream. Of Mice and Men shows that for poor migrant workers during the Depression, the American Dream became an illusion and a trap. All the ranch hands in Of Mice and Men dream of life, liberty, and happiness, but none ever gets it. As Crooks says when he hears of Lennie's dream to own his own farm, "Nobody ever gets to heaven, and nobody gets no land."
At the same time, while the dream may never be realized, Of Mice and Men suggests that in order for life to be full and meaningful, it must contain dreams. George and Lennie never achieve their dream, but the dream holds their remarkable friendship together. Their dream is real because it's real in their imaginations. The dream keeps Lennie happy and stops George from becoming "mean" and lonely like most ranch hands. The dream gives them life, even if life never allows them to achieve their dreams.