Why did Steinbeck stop "The Grapes of Wrath" with the scene in the barn?

Once of the main themes of the novel is that men need to work together if they are to succeed in overcoming the things that threaten them. This idea is first seen in Casy's idea of the oversoul in which he states that his belief that man doesn't have an individual soul but he his connected to others in a large "oversoul'. Thus, one person's problems become the problems of others. The tractor driver, Davis, first violates this idea when he tractors over Muley's house, even though he is the son of one of the sharecroppers. Davis uses the excuse that he has to "take care of his own" first and others have to take care of themselves. As the novel continues, Casy eventually loses his life trying to help the migrant farm workers earn a decent wage. Tom learns from that and leaves the family in order to continue Casy's work. He tells Mama that she won't have to worry about him because whenever injustice is being committed he'll be there. He has come to believe that people must help other people. His sister, Rose of Sharon, is a different case. All during the novel, she seems more concerned about herself and "getting milk" so she can deliver a healthy baby. Unfortunately, her baby is stillborn. However, she seems to have also learned that people must care for each other. In the barn, she is the only one who can save the man who is dying from starvation. She still has milk in her breasts that would have fed her dead child. By offering to nurse a grown man, she shows that she now understands, as Tom did, that people must help each other in order to survive. Thus, the main characters of the novel have changed and now understand and agree with the theme Steinbeck has been trying to convey during the entire novel: the common man must sacrifice and work together if he is to overcome all the injustices and threats to his survival. Rose of Sharon's act of feeding the starving man is an unexpected but appropriate place to end his novel because one of the main characters who has been so selfish during most of the novel commits a truly unselfish act.