The Robert Burns poem "To a Mouse" contains a stanza which was the reason behind the title of the story "Of Mice and Men".
The best laid schemes o' mice an' men
Gang aft agley,
An' lea'e us nought but grief an' pain,
For promis'd joy
In regular English it would read:
The best laid schemes of mice and men
Go often askew,
And leaves us nothing but grief and pain,
For promised joy!
The relationship is precisely that George and Lennie had "best laid plans" together. They dreamt of running their own farm, and to be left alone. They wished to get out of being miserable and invisible to the world. They did not plan to hurt anyone in the process, if anything, all they wanted was a chance to make a better life.
Yet, as you can see in the story, their best laid plans went nowhere. Lennie ended up dead, George's dream disappeared in front of him, and all the hopes they felt they deserved to at least dream of, were taken away by a realistically cruel society.