What is Steinbeck saying about dreams? Is the book's take-home message inherently pessimistic one? should we all just give up and stop dreaming?

I think dreams in this book are specific to the context of Steinbeck's world. The great depression caused widespread havoc among the population. Jobs were hard to come by and dreams were in very short supply. One could extend some of the themes to life in general bit I would be hesitant to claim that dreams cannot be realized.