In Coleridge's "The Rime of the Ancient Mariner," why was the mariner narrating the story to the guest?
The mariner committed a sin against one of God's creatures by killing a friendly, harmless albatross, and the polar spirits punished him for it and required him to do penance during and after the voyage. At the end of his tale, the mariner explains in lines 581 - 585 that ever since he made it safely back to his own harbor, he has a spasm of agony "at an uncertain hour." Whenever that pain strikes him, he cannot rest until he has found the appointed person who needs to hear his story and relates the entire event to him. The man goes from place to place, and as soon as he sees the man who needs his story, he knows it. The mariner's speech is enchanted; the person hearing it is under a spell and is unable to leave until the whole story comes out. That is why the wedding guest missed the whole ceremony of his relative who was getting married. As much as he wanted to attend the wedding, the mariner's bewitching tale held him captive. After hearing the mariner's account, the wedding guest is finally able to go his way, and he heads home. The next day, and presumably thereafter, he was "a sadder and a wiser man." He evidently learned the lesson he needed to learn.