Why does King Alfonso want Ana to marry Juan?

King Alfonso wants to arrange a marriage between Don Juan and Ana. He talks things over with Ana's father, Don Gonzalo, who thinks it's a good idea. They hope that by marrying Ana, this notorious rake and libertine will finally settle down. But a leopard can't change its spots, and a reckless womanizer like Don Juan is constitutionally incapable of adjusting to the responsibilities of married life.
Word reaches the court of Don Juan's devious seduction of Duchess Isabela. King Alfonso feels like a complete fool; he imprisoned Isabela and Duke Octavio until they were married as Octavio was alleged to have crept into Isabela's bedroom and made love to her. It was in fact Don Juan impersonating Octavio, and the revelation of this uncomfortable fact infuriates the king. He reacts by calling off the proposed wedding and banishing Don Juan from Seville.