The ancient mariner tells his story to the wedding - guest. He does this rather abruptly, seizing the wedding - guest as he is on his way to the festivities. This act of the mariner is quite intrusive, yet it sets the stage for the mariner to recount his adventures at sea. The wedding - guest is initially reluctant, but as the mariner begins his tale, he becomes transfixed. It's as if the mariner has chosen this particular person randomly, but in the context of the story, it could be seen as a way for the mariner to share his experiences and perhaps seek some form of absolution through the telling of his story to someone new and unsuspecting.
The ancient mariner tells his story to the wedding - guest.
Well, the ancient mariner tells his story to a wedding - guest. You see, the mariner stops this wedding - guest who is on his way to a wedding celebration. It's quite an unexpected encounter. The mariner seems compelled to share his strange and harrowing tale with this stranger. Maybe he needs to unburden himself, or perhaps there's some other reason that we can only guess at. But the wedding - guest has no choice but to listen as the mariner begins to unfold his extraordinary story.