What causes Uncle John's guilt about his past in The Grapes of Wrath?

Uncle John's guilt has a single source.

His sense of guilt causes him to blame all the family’s misfortunes on what he thinks of as his sin: his failure to call a doctor when his wife complained of illness.

He feels that he is responsible for his wife's death. When he was a younger man his wife was sick during her first pregnancy. He told her that she was not truly sick and that she would feel better in the morning. 

In the morning she died.

Uncle John holds himself responsible for this death and tries to make up for his guilt as the years go by. This trait in him suggests that he is a good man at heart. He learned to sympathize with people through this hardship. 

Another thing we see in his character is a strain of "absolutism". He refuses to forgive himself, though he did not cause his wife's illness and he has attempted to make up for what he thinks of as his mistake for years. 

In The Grapes of Wrath, why does Uncle John have guilt that is driving himself crazy?  

Uncle John is haunted by the death of his young wife many years before. They had been married only four months. She had been pregnant. When she complained of pain in her stomach, John dismissed it, saying she had eaten too much. She died the next afternoon. "[S]omepin jus' bust in her. Ap-appendick or somepin," Tom says. Uncle John's guilt over her death consumed him. Tom explains it in Chapter 8 of the novel:

. . . Uncle John, he's always been a easy-goin' fella, an' he takes it hard. Takes it for a sin. For a long time he won't have nothin' to say to nobody. Just walks aroun' like he don't see nothin', and he prays some. Took 'im two years to come out of it, an' then he ain't the same.

His guilt changed him and directed the rest of his life.