The source of the characters' dreams is their discontent with their present.
Steinbeck shows how poor their lifestyle is: they have few possessions, fewer comforts, no chance of marriage or family life and no place of their own.
George and Lennie's dream is the main dream in the novel, They dream of owning their own land, and being able to be their own boss, they want freedom and the ability to do what they want when they want.
Lennie dreams of tending the rabbits and "Living of the fatta the lan'".
Many dreams in the work have a physical dimension: Not just wishes to be achieved, they are places to be reached.
The fact that George’s ranch, the central dream of the book, is an actual place as opposed to a person or a thing underlines this geographical element.
At every stage of the story John Steinbeck hints at trouble even from the start there is an element that something will go wrong as one of the first things you hear about Lennie is that he doesn’t remember where or what he has been doing and George has to remind him, in the first chapter George and Lennie are hiding from angry mobs of people this being because of Lennie and his child like ways.
The hope that Lennie and George can achieve there dream of owning there own ranch is reiterated throughout the story at every stage of the story there is a small hope of this dream that George and Lennie have of owning there own ranch, they continuously look to forward to the future because of this dream that George creates for Lennie to keep out of trouble, in a way it is sort of bribery as George tells Lennie if he doesn’t behave himself then he can’t tend to the rabbits.
But by the end of the story, Steinbeck reveals that dreams can be as poisonous as they are beneficial.
What George discovers—and what Crooks already seems to know when he scornfully spurns Candy’s offer to join him, Lennie, and George—is that dreams are too often merely an articulation of what never can be.