Why did Steinbeck decided to name his book "Of Mice and Men"?

The title of John Steinbeck’s novel “Of Mice and Men” comes from a Robert Burns poem entitled “To A Mouse”.  The poem’s speaker is talking to a mouse, basically explaining to him why he is afraid of the speaker and why he is so small and weak – this could definitely relate to the character of Lennie or could represent George talking to Lennie as the speaker talks to the mouse.  The actual title comes from Burns’ stanza which states,

“But Mouse, you are not alone,
In proving foresight may be vain:
The best laid schemes of mice and men
Go often askew,
And leaves us nothing but grief and pain,
For promised joy!

This stanza directly relates to the situation that the two men are in throughout the novel.

They have a plan to buy their own farm and “live off the fatta the land.”  And even though they dream of this every day of their lives and make attempts to make this dream come true, the events that take place which are out of their hands do not allow this dream to happen – therefore, bringing true the last four lines of this stanza.