Where did the name Of Mice and Men come from?

The title of John Steinbeck's novel Of Mice and Men is taken from the poem of Robert Burns, "To a Mouse, On Turning Her Up in Her Nest with a Plow," written in November of 1785. In that poem, Burns writes that
The best laid schemes o' mice and men
Gang aft a-gley [often go astray],
And lea'v us nought but grief and pain,
For promised joy.

At the beginning of Of Mice and Men, we learn about George and Lennie's dream (or scheme) of owning their own ranch, living off the fat of the land, and raising rabbits. Just when it appears that George and Lennie will get their dream farm, fate steps in, and those plans go astray.

In this case, fate is given a hand by Lennie's inability to control his strength and to understand what to do. In the end, George is left with the "grief and pain" of losing his friend Lennie and therefore of losing the "promised joy" of running their ranch together.