Question

Why

Why had the old mariner to tell his story to one of the wedding guests

Answer 1

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. 

Answer 2

The old mariner had to tell his story to one of the wedding guest because he is compelled,  perhaps by the metaphysical,  to tells his tale so others do not make the same errors as he did.