The basic message that John Steinbeck wishes to bestow upon the readers in his novel Of Mice and Men is best expressed in the stanza from which the idea of the title of the novel came.
The best laid plans of mice and men often go awry
and leave us nothing but grief and pain
for a promised joy.
The title Of Mice and Men comes from a verse taken from Robbie Burns's poem "To A Mouse". It is a poem dedicated to a mouse who keeps trying to make a living within the wall of a home and does his best to at least make a home for himself.
Similarly, John Steinbeck uses this poem to illustrate that the home that George and Lennie dream about, and their own American dream, are as easy to vanish into nothing as they are easy to come true. Hence, no matter how heavily one plans, or dreams, dreams end up being merely abstracts of our hopes.
Conclusively, the general theme of the novel is that dreams may not always come true, even if we try hard and stick to a plan. Success only happens when all circumstances are favorable, and are in place. Dreams do not come true. They are only dreams.