Your instructor's statement with respect to the "apparent emotional peace and stability" Mary Rowlandson describes at the end of her narrative gets to the heart of two important elements in the Puritan captivity narrative: the lessons the captive learns about God's providence and the changes within the captive as a result of the experience.
Mary Rowlandson, the wife of a Puritan minister in Lancaster, Massachusetts, was captured in a Wampanoag tribal raid on February 20, 1676 and spent eleven weeks with the Indians, having been ransomed in Princeton, Massachusetts, on May 2, 1676. One of her three children, Sarah, died of wounds during the captivity. When Rowlandson was ransomed, her two surviving children were still captives but were released soon after Rowlandson's release.
During the period in which Rowlandson was taken captive, American Indians generally took captives for several reasons: captives were used for ransom; as slaves; to be sold to other tribes or to the French; and as adoptees to replace members of the tribe lost through age or battle. Rowlandson and her children, because they were an intact family unit, were ideal prisoners for ransom. When Rowlandson was captured, her husband was away from home to arrange, ironically, for the protection of Lancaster from Indian incursions.
In her concluding remarks of her narrative, Rowlandson describes the remarkable turn that her life has taken as a result of the captivity:
I have seen the extreme vanity of this World: One hour I have been in health, and wealth, wanting nothing. But the next hour in sickness and wounds, and death, having nothing but sorrow and affliction.
As a Puritan who searches for meaning in experience, Rowlandson is examining her captivity to see what, if anything, she has learned about herself and her relationship to God. The captivity experience, which turns her life upside down, is only a disaster if she fails to understand what that experience is meant to teach. When she says that "I have seen the extreme vanity of this World," she means the material, comfortable elements of life that can be instantly converted into "sorrow and affliction." In the Puritan belief system, an experience—good or bad—is God's way of instructing his people in how to behave in the world, and the failure to understand an experience is a lost opportunity to grow spiritually. Rowlandson now understands in a visceral way that the comforts of this world are truly transitory.
Before her experience, Rowlandson notes that "I used to sleep... without workings in my thoughts"— that is, with no worries or deep thoughts. After the captivity, however, she writes,
my thoughts are upon things past, upon the awful dispensation of the Lord towards us; upon his wonderful power and might, in carrying us through so many difficulties, in returning us in safety, and suffering none to hurt us.
Rowlandson's captivity experience, which could have ended in her death or enslavement, places her in the unique position of being able to prove—by her very existence—God's power and "awful dispensation"—that is, God's majestic kindness in shielding his people from disaster. In other words, faith in God's goodness is made tangible in the lives of Rowlandson and her children. God's intervention on her behalf illustrates his ability in the physical world to protect his people, a lesson that is not lost on Puritans who believe that good and evil are active forces that can affect their daily lives.
Rowlandson concludes her narrative with an almost modern sentiment:
If trouble from smaller matters begin to arise in me, I have something at hand to check my self with, and say, why am I troubled? ... I have learned to look beyond present and smaller troubles, and to be quieted under them....
The trials of her captivity experience, including the death of her child during captivity, have given her a new and more expansive view of what is important and unimportant in this life.