The title 'Of MIce and Men' is taken from a Robert Burns poem, and is commonly thought to mean that even the most carefully prepared plans may go wrong.  Some people interpret Steinbeck's meaning to be that the weak, Lennie, Candy, Crooks and Curley's wife, are represented by the mice, and the strong, George, Slim, Carlson, are represented by the 'men'.  Ultimately, the dreams that these people have do not go according to their plans.  The pessimism of this idea reflects the 'mood' of the time which was fed by the Depression.

The best laid schemes o' mice an' men
Gang aft a-gley,
An' lea'e us nought but grief an' pain,
For promised joy!

The best laid schemes of mice and men
Often go wrong
And leave us nothing but grief and pain,
Instead of promised joy!

Robert Burns 1875