It is important to remember that this novel is written in a very different time when people held (by our standards) very traditional religious views that shaped their outlook on life and on many different aspects. In Chapter Seventeen, when we are presented to Nancy after a break of sixteen years, and see that she is now married to Godfrey Cass, we are shown her thoughts about adoption.
Of course, Godfrey's idea to adopt, and in particular to adopt Eppie, stems from their inability to have any children of their own and also Godfrey's guilt at not claiming his rightful child as his own. Now that his father has died, Dunstan remains disappeared and Nancy is his wife, he feels able to give his daughter, Eppie, the rightful position in society that ironically she actually deserves. However, it is Nancy that disagrees:
To adopt a child, because children of your own had been denied to you, was to try and choose your lot in spite of Providence: the adopted child, she was convinced, would never turn out well, and would be a curse to those who had wilfully and rebelliously sought what it was clear that, for a some high reason, they were better without. When you saw a thing was not meant to be, said Nancy, it was a bounden duty to leave off so much as wishing for it.
Thus, from Nancy's perspective, trying to adopt because you can't have your own children is trying to interfere in God's providence and the "high reason" that had made you unable to bear children. Interfering with Providence, from Nancy's perspective, can only bring trouble as you reject what God has for you.