Why did Rose get angry at Will when he read the letters?

In Medicine River, the trunk of letters that Will's mother Rose keeps in their home is not merely a historical archive about her relationship with her husband, Bob. It is also a repository of historical trauma that she wants, at first, to keep sealed off. When she beats Will after she discovers he has been reading the letters, he struggles to understand her anger, believing that it is only natural to want to recover one's missing family history. At the end of the novel after contextual information emerges about their interracial marriage and family opposition, Will realizes that Rose wanted to protect him from internalizing the narrative of his father's loss, especially the fact that he died in a car accident.