why the wedding guest listens to the mariner like a three years child??

From the Ancient Mariner's first interaction with the Wedding Guest, we know there is more to him than the fact that he appears unnaturally old. He has a "glittering eye" that immediately unnerves the Wedding Guest, who presumes he is mad and calls him a "grey-beard loon." Yet there is more to his "glittering eye" than mere madness, as he is able to compel the Wedding Guest to listen to his story with the fascination of a three-year-old child. Although he is clearly human, the Ancient Mariner seems to have a touch of the otherworldly in him. The Mariner is compelled, perhaps by the metaphysical, to tell his tale so others do not make the same errors as he did.