What is the significance of dreams?

Many of the characters in Of Mice and Men, enveloped by the grayness and desperation of their lives, seek a glimmer of sunshine in dreams. George recites for Lenny the "dream." After the discovery of Curley's dead wife, Candy expresses his "greatest fear" that the dream of owning a farm has expired. George stoically reflects, "I think I knowed from the very first. I think I knowed we'd never do her [get the farm]. He [Lennie] usta like to hear about it so much I got to thinking maybe we would." When the dream of a ranch forms itself in the minds of Lennie and George, both men become energized by the hope of escaping their trapped lives. Without this dream as a satisfying and motivating notion, they sink into hopelessness and despair.
This necessity of dreams exists in even the poorest of hearts--perhaps more so than in the hearts of others. To dream is intrinsic to the human condition, as poet Robert Browning writes in "Andrea del Sarto":
Ah, but a man's reach should exceed his grasp, Or what's a heaven for?