Perhaps the moral of Steinbeck's novel, "Of Mice and Men" is an existential one. The "best laid schemes o' Man an' Mouse," do go "awry" as Robert Burns wrote in his poem because the brotherhood of men was never so shattered as in the Great Depression when men left their families either out of despair as Arlo Guthrie did, or when they left in search of work. At any rate, they were lonely, desperate men in this time, isolated misfits.
In his novel Steinbeck shows that humans cannot live in isolation without consequences. Steinbeck seems to be saying " I regret that Man's dominion has broken Nature's social union" when George is forced to shoot his friend Lennie.
Steinbeck's belief in the interdependence of society is a theme he explores throughout the novel. Often Slim with his "God-like eyes" is the one who intervenes in situations. He is the one who consoles Candy when his dog is shot, telling him he can have one of his dog's pups. Slim is also the one who consoles George after Lennie's "mercy" killing and takes him in as a friend: "you hadda, George. I swear you hadda. Come on with me."
An excellent question, and one certainly open to interpretation. "The Best Laid Plans" perhaps?
George and Lenny have their dreams, but in the end it doesn't matter. George's dreams of a farm crash when he puts a bullet in Lenny's head. The accidental murder of Curely's wife, an act seemingly unavoidable (foreshadowed by Lenny's killing the various small animals earlier throughout the book) given Lenny's treatment of things that are "soft". Neither man is bad, but Lenny's nature dooms them from the start. George tries to protect Lenny, mostly from himself and others, but in the end the childhood (Lenny and his innocence) must give way to adulthood (George).
Other lessons might be "having to put aside childish dreams", "Nothing is more destructivethan a child", and possibly "Dreams do not always come true."
You are not guaranteed success until you succeed.