The best laid schemes o' Mice an' Men,
Gang aft agley,
An' lea'e us nought but grief an' pain,
For promis'd joy!
This is half of a poem by the Scottish poet Robert Burns that from which Steinbeck took his title, and I'll paraphrase the four lines because they're written in a Lowland Scots dialect: the best plans of mice and men often go completely wrong, and leave us nothing but grief and pain instead of promised joy.
George and Lennie's plans for a place of their own are certainly worth pursuing, and there is absolutely nothing wrong with the plan itself. The disasters that occur that keep the plan from coming to fruition are caused, in large part, by outside forces beyond George and Lennie's control. Unfortunately, Lennie's mental deficiencies also cause perhaps the most critical disaster--the accidental killing of Curley's wife--and that act brings nothing "but grief an' pain" to George and Lennie.
Burns' poem is a perfect epigram for Steinbeck's novella.
The general impression of the title Of Mice and Men is that it is borrowed from Robert Burns's poem "To a Mouse" and refers to the plans that George and Lennie have to own their own little subsistence farm. This is explained perfectly in the first answer to the posted question. However, the title is probably intended to suggest a second meaning as well, and the second meaning might be more important in expressing Steinbeck's philosophy than what might be called the overt meaning concerning George and Lennie.
Steinbeck's title seems to be implying that all the working men in the novel are mice. They are as small and weak and helpless as the little dispossessed mouse in Robert Burns's poem, and they have no more economic security than that Scottish mouse whose home was accidently destroyed by the poet's plow. What is true of the working men in Steinbeck's novel is, in his view, true of all working men all over the world. They all have their hopes and dreams, but most of them are condemned to hard labor, low wages, and insecurity for the rest of their lives. Nobody cares what happens to any of them.
Steinbeck does not confine his message to George and Lennie. He shows how Candy is even more insecure than the others because he is an old man and he lost a hand in an accident and how Crooks is more insecure than the others because his race bars him from most employment opportunities and forces him to take even lower-paying jobs than the others. Candy and Crooks are even more like mice than George and Lennie.
All the workers in Steinbeck's novel live in one big bunkhouse. They could all get turned out of that shelter and they could all lose the food they are given in the mess hall without warning. If there was a crop failure, an economic downturn, or if they were not needed for a few cold winter months because all the work had been finished, they could all be turned out and become bindle stiffs on the roads. George and Lennie were bindle stiffs until they got to this ranch and were assigned a couple of bunks, and George will go back to being a bindle stiff until he finds another ranch to take him in.
I think it's because there is a leader and a follower and the leader is the man and the follower is the mouse.