Repeated lines often refer to a specific theme or concept that the author wants to be clear. "I fear thee ancient mariner" refers to the Wedding-guest's fear that the Mariner is supernatural. Before the mariner finishes his story, the guest believes that he is a ghost. The mariner must calm these fears. The theme to which this points is the supernatural nature of the story.

I believe the other line that you are asking about is "he cannot choose, but hear." This shows that even though the wedding guest is frightened and slightly disturbed, he cannot leave because he is enraptured and captivated by the story. This line also points to the supernatural effect that the mariner has over the Wedding-Guest.

The repetition of the lines show the importance of the supernatural demeanor that the mariner has.

Coleridge repeats the lines for emphasis, to show how deeply involved the wedding guest is in the story being told. The guest can hear the wedding beginning, but is rooted to the spot by the tale the mariner tells. Not the music, the laughter, or the sound of the guests' voices can tear him away from the sad and eerie tale. As the guest listens, he fears the mariner might be an evil spirit or ghost bent on taking his soul. The mariner tells the listener he is neither to soothe his fears and keep him listening. The guest must have something in common with the mariner that the old sailor must save him from by telling his tale of woe. The wedding guest is saved from the mariner's fate when Coleridge tells the reader the guest arose a sadder but wiser man.