In act 3, scene 1 of William Shakespeare's The Tempest, Prospero watches unobserved from a distance while Ferdinand and Miranda profess their love for one another and resolve to be married—just as Prospero hoped they would.
Prospero says that everything he does is for Miranda's benefit.
PROSPERO: I have done nothing but in care of thee. (1.2.19)
For the most part, this is true. Prospero brought Ferdinand and Miranda together so they would fall in love and Miranda would be happy, but Prospero also has ulterior motives. He hopes that the marriage of his daughter to Ferdinand, the son of Alonzo, the King of Naples, will help restore Prospero to his former position as Duke of Milan.
This is all part of Prospero's larger plan to take revenge on those who usurped his dukedom and cast him adrift with three-year-old Miranda twelve years ago.
However, Ferdinand and Miranda's inherent goodness and their caring relationship demonstrates the kindness and humanity of one person towards another, and this tempers Prospero's desire for revenge against the people who wronged him.
Prospero confronts his brother, Antonio, for usurping his dukedom, and he criticizes Alonzo for assisting Antonio in his usurpation. Due to Ferdinand and Miranda's positive influence on him, however, Prospero forgives Antonio and Alonzo and restores their ship so that everyone can sail back to Naples for Ferdinand and Miranda's wedding.