What is the back story as to why Steinbeck gave Of Mice and Men that title?

John Steinbeck's choice of a title is an allusion to a well-known poem by Robert Burns, the full title of which is "To a Mouse, on Turning Her Up in Her Nest with the Plough." The speaker in Robert Burns' touching poem expresses regret for inadvertently destroying a mouse's nest while plowing. In the next to-last stanza he says:

But Mousie, thou art no thy lane,In proving foresight may be vain:The best-laid schemes o' Mice an' Men                       Gang aft agley,An' lea'e us nought but grief an' pain,                       For promis'd joy!

The title derived from Robert Burns' poem alludes to the dream of George and Lennie to own their own little farm and to be free of the backbreaking toil and wage-slavery in which they are presently trapped. This dream is the back story. It is repeated to Lennie by George in the first chapter. Lennie never gets tired of hearing it. Later in the story it seems that the dream may actually come true because Candy volunteers to contribute $300 to buying a farm George knows can be had for a total of only $600. Then everything goes wrong and Lennie has to flee and hide by the river where they camped in the first chapter. George is planning to shoot his friend to protect him from a worse fate at the hands of a lynch mob; but in the last chapter, at Lennie's request, he tells him part of the dream again.

"We'll have a cow," said George. "An' we'll have maybe a pig an' chickens . . . an' down the flat we'll have a . . . little piece alfalfa------"

But George can't continue with his dream-story. This is only another instance in which the best laid plans of mice and men often go astray. The lynch mob is fast approaching. George must pull the trigger of the stolen Luger and kill the only friend he ever had.