It is interesting that when Lennie urges George to recount the dream of owning their own land the antagonism that George feels towards his simple friend disappears. Recounting the dream draws them closer and gives them something they can both hope for. At one time it does also seem attainable when they talk of pooling resources with Candy. This is in contrast to Curly's wife who only seems to feel regret at what she might be/could have been. It leads her into spiteful behaviour that ultimately endangers her life.
In the desperate times of the Great Depression, those men who were forced by poverty to leave their homes in search of work had nothing else but the American Dream to which they could cling. For, these dreams are what give the men hope of a better life; a life that is more meaningful.
Curley's wife's dream is one of regret rather than hope. For, she realizes the futility of hoping that she will be an actress. When she speaks of her dream, it is with rue.
Both Lennie and Curley's wife will never see their dreams achieved. George feels sorry for Lennie because Lennie will not see his own dreams come true. As for Curley's wife, not many people knew about her dreams. The fact that she dies, with her dreams left to be full-filled, only supports the fact that everyone simply ignored her (she is so far removed from the lives of the others characters that she is not even named).
In addition, these dreams help to emphasize to us how downtrodden everyone is in this time and place in American history. Everyone has a dream that they are trying to reach for, but none of them has a real shot at reaching that dream. Steinbeck is being very negative about the true state of America.
Curley's wife's dream of becoming a famous movie star is suggestive of at least two things: 1) she is bound to be disappointed with the reality of her life and 2) she is just as conventional as the rest of the characters in terms of their world-views.
The dream that Lennie and George share is a touching yet somewhat stereotypical dream, perhaps it draws its power from its very commonality. It bears the logic of the obvious. Of course they want to own a place of their own and work less. Everyone wants that.
These dreams serve to connect the audience with the characters and to elicit sympathy as well, as the audience can both relate to the dreams and see them for what they are - innocent visions of a better future which are unlikely to materialize.
Lennie and Curley's wife both have impossible dreams. Lennie dreams of having a farm, where he and George answer only to themselves and Lennie can tend the rabbits. Curley's wife wants to be noticed. She says a man once told her she could be a movie star.