Why is the ending of "The lady, or the Tiger?" So effective? How does this ambiguity affect the reader?

The princess knows the answer and through the reading of this novel and by understanding the character development of the princess the reader is pushed to make his or her own assumptions and conclusions about what she would have chosen for the young man. As well, the author attempts to impose self-reflection and perspective. For example, a man who comes from drought and finds it raining would be happy, a man who comes from a rainy place, might find the rain miserable. Was the princess an outright sociopath or killer? Or through her interest in the young man, was she self-sacrificing for the benefit of others? That is the conclusion the reader must draw on based on his or her own experience with the story and perspective.