Dreams are the driving force behind the lives of many of the characters in Of Mice and Men. In the case of some of these characters, dreams are really all they have left to keep their hopes alive.
The dream that we are made aware of right away, is the dream of George and Lennie.
O.K. Someday—we're gonna get the jack together and we're gonna have a little house and a couple of acres an' a cow and some pigs and—" "An' live off the fatta the lan'," Lennie shouted. "An' have rabbits.
The dream is so clear in the minds of both men that they can even recite it. Sadly for them, this dream has been alive for too long. The more they get in trouble, mainly because of Lennie, the more distant the dream becomes.
In fact, shortly before George kills Lennie, out of mercy and in order to avoid a lynch mob from Curley's people, he asks Lennie to visualize the dream one more time, right before shooting him. This is symbolic of the end of the dream altogether. It is now officially over, not just for them, but for all of the other sad men who ended up believing in it, too. Hopes are dead as well.
Still, George and Lennie's dream inspired other people. In chapter 3, as George and Lennie discuss their dream again, they are overheard by Candy. Candy not only is touched by the dream, but wants a part of it as well. He even offers his saved money to make it happen.
It is interesting, however, to note how George and Lennie react when they learn that Candy has heard about the dream.
At first, they jump, as if they had been caught doing something wrong. This shows the extent to which the two men really, deep inside, know that the dream is one of great magnitude. They are fully aware of how huge it is. However, they find comfort and peace in visualizing it. This is why they feel, when Candy "finds them out," as if they have been caught doing something inappropriate.
In chapter 4, it is Crooks who buys into the dream of George and Lennie. After insulting it first, laughing at it, and trying his best to bring the men's hopes down, he suddenly changes his mind
[Crooks] hesitated. "… If you … guys would want a hand to work for nothing—just his keep, why I'd come an' lend a hand. I ain't so crippled I can't work like a son-of-a-bitch if I want to..
The dream, in Crooks's case, shows that even the more torn-apart men, and even the most broken of them all, can still be put back together again with a tiny speck of hope.
Curley's wife is another character whose dream helps her stay "put together" even in the dire environment of the farm. Her dream of making it in show business was the motivating factor that kept looking pretty and presentable, rather than living her life as a frumpy farm wife. Sadly for her as well, her dream is too far from her. She is now married to Curley, dissatisfied with her life, and very bored. Her dream is the only motivation she has to tolerate life there. She even talks about her dream on her very last day of life, as she is talking about it with Lennie, and right before he accidentally kills her.
In all, dreams and hopes are almost interchangeable in the novella. The characters renew their hopes for a better life thanks to the dreams that they hold dearest to their hearts. Sadly, none of them gets to realize their dreams. Still, they do not stop bringing them up and thinking about them.