What is the overall message in Steinbeck's Of Mice and Men, given that the author continuously illustrates friendship, commitment, and relationship throughout the novel?

Of Mice and Men conveys how important it is for human beings to care for one another, especially in difficult circumstances, even though we understand the limited effects of friendship and affection on society at large.
John Steinbeck shows the relative advantage and disadvantages of all the characters, even those who consider themselves superior to others. Curley is arrogant because he is the boss’s son, but he is unkind to others. Lennie’s physical strength allows him to earn a living through hard work and to dominate Curley physically, but that strength is offset by the disadvantages of his intellectual disabilities. George has learned about kindness through his relationship with Lennie, including the shame he felt when he formerly tormented him for entertainment. George’s soft heart is an impediment to his own well-being, however, as he continues to partner with Lennie rather than strike out on his own.
The harsh social realities of the Depression, when work was scarce, exacerbate the inequalities in the socio-economic system. In such challenging times, the men’s efforts to befriend each other were circumscribed by the competition over jobs and their constant fear of being dismissed. The friendships the men formed, no matter how tenuous or temporary, represent for Steinbeck a triumph of the human spirit over the dehumanizing effects of a system that had little or no safety net.

Steinbeck does illustrate the importance of friendship, commitment, and relationships throughout the novel. Although not a revolutionary and not a member of the Communist Party, Steinbeck was sympathetic to communist ideas. Therefore, community building and solidarity are, in his opinion, foundational to building a good society
Steinbeck, however, offers a grim ending to Lennie and George's dream of building a community around owning their own farm. They have the right idea, and many of the ranch hands respond positively to the dream, but Steinbeck is trying to show that two ordinary, downtrodden men cannot individually combat the crushing boot of capitalism. Because Lennie has a handicap and because the two are forced, as migrant workers, to wander from one job to another, it is difficult for them to find a place where they can fit in. The system sees George and Lennie not as fully human but as widgets in the profit-making agricultural machine. The system doesn't see any need to make accommodations to help Lennie, and this leads to disaster. Without Lennie, George is overwhelmed by feelings of hopelessness, which means he will blow all his earnings and stay in bondage to low-wage, insecure jobs.
The overall message is that the conditions of the 1930s betrays the American Dream and crushes and destroys individuals.

Friendship and commitment are at the heart of this story and are shown most clearly through the relationship between Lennie and George. Through these two characters, Steinbeck presents his central message: friendships are often strengthened by difficult conditions, like poverty and powerlessness, but they cannot always overcome such tests.
From the very beginning of the story, it is clear that their friendship is based on unequal footing. George, for example, is like a father figure to Lennie, and Lennie relies on George to take care of him. Despite this, they are united (and strengthened) by their dream to own their own ranch and live off "the fatta the lan'." This causes some suspicion and interest from other characters in the story: remember that in Chapter Two, for instance, Slim comments that not many men travel around together. But this attitude does not affect George and Lennie: they are more determined than ever to achieve their dream and make a successful life together. Even when Curley takes an instant dislike to Lennie, their friendship holds strong and their commitment to each other is unaffected.
George and Lennie's friendship (and their dream of owning a ranch) is so strong that it attracts other characters in the story. Both Crooks and Candy, for example, are drawn in by the dream of owning a ranch and plan on becoming part of George and Lennie's brotherhood.
But when Lennie kills Curley's wife, it becomes clear that no amount of friendship or commitment can save him. Even though her death was accidental, George cannot stop the men from hunting him down and shooting him. This not only brings their friendship to a tragic end but also makes it clear that these men will never overcome the poverty and powerlessness which is inherent in their lives.