"Duty" is a word repeated over and over in Ghosts. Everyone has duties related to the role they play in the society. The wife/mother is duty-bound to protect her husband's reputation (no matter how unsavory he is) and to sacrifice everything for her son. In return the son must love his mother. The maid is duty-bound to clean up everyone's mess – no matter how colossal it is. The Pastor is duty-bound to regulate everyone else's fulfillment of their duty. No wonder these people seem so dour. All this responsibility is exhausting and keeps them from being themselves.
Pastor Manders represents a necessary duty-bound segment of society?
In her last scenes with Oswald, Mrs. Alving fulfills the accusations of self-interest Pastor Manders made in Act 1.