How does Lennie and George's friendship propel the overall message that Steinbeck illustrates about friendship, commitment, and responsibility?

As far as friendship is concerned, Lennie and George's relationship is contrasted with that of other ranch hands. Their friendship is a rarity within this context and makes the two men different. The depth of their acquaintance is best illustrated by the fact that George takes responsibility for Lennie's actions. He does not alienate himself from Lennie and make him face retribution for the supposed wrongs that he commits alone; rather, George shares his trials and tribulations with him. In the incident at Weed, for example, he assists Lennie in his escape even though he himself has not done anything wrong. They both manage to evade their trackers by spending hours in a ditch. George could have easily dumped Lennie and avoided persecution, but because he felt responsible for the poor soul, he helped him get away.
Even though George expresses frustration at Lennie's actions, he still supports him and encourages him by consistently sharing their dream with him. Steinbeck illustrates through George's actions the true nature of friendship. It is clearly about sharing both good and bad with the one you are friends with.
George's relationship with Lennie also clearly requires patience and commitment. Since Lennie is essentially oblivious to the outcomes of his actions and, therefore, does things without thinking, George, as the rational partner, has to continuously educate Lennie about what he does and its possible result. He has to do this repeatedly and even though he at times expresses his exasperation, he persists. George took it upon himself to take care of Lennie when he promised Aunt Clara that he would take him under his wing. George can be praised for keeping his promise; he does not once betray his friend or abandon him.
George has taken on a huge responsibility. He realizes that Lennie would be lost without him, even though Lennie claims that he can survive alone, as in the following conversation:

"George, you want I should go away and leave you alone?""Where the hell could you go?""Well, I could. I could go off in the hills there. Some place I'd find a cave." "Yeah? How'd you eat? You ain't got sense enough to find nothing to eat."

George acknowledges and apologizes to Lennie for being mean and later tells him that he doesn't want him to go. It is clear from the conversation that they are truly dependent on each other and care about one another. This bond is best epitomized by their discussions on buying some land. George enjoys telling Lennie how they are different because of the fact that they have someone to talk to and turn to; they are not lonely, and they have something to look forward to.
The juxtaposition of what George and Lennie have with the situation of the other ranch workers clearly illustrates the fact that friendship and commitment in the kind of society they live in counts. It provides those involved in such relationships the security and confidence that one needs to survive in such a harsh world. It gives one's life a sense of purpose and meaning. Without friendship, life is empty and lonely. It is a pity that George and Lennie's acquaintance ends so abruptly and cannot run its full course.