Why did John Steinbeck called his book Of Mice and Men?

After a poem by another author called "To a Mouse" written in 1785 by Robert Burns, a Scottish poet. The line is taken from the second to last stanza.

But, Mousie, thou art no thy lane,

In proving foresight may be vain;


The best-laid schemes o' mice an' men

Gang aft agley, 40


An' lea'e us nought but grief an' pain,

For promis'd joy!