The Great Imposition
Imagine that you take a total stranger and, without so much as a by-your-leave, set him down in a strange place, attach him for life to a host of people he knows nothing about, and proceed to control every aspect of his life. For a month or more thereafter he spends his time (when not sleeping) either staring glumly at you or weeping in despair.
With that picture in mind, you can understand why a baby's beginning to smile is greeted with such eagerness. At last his parents have some hope that they are not creating unabated misery in the child's life.