What does the poet mean?

What the reader finds in The Ancient Mariner is one thing. What the author wishes to tell with the poem, if he wishes to tell anything at all, may be something different. It is hard to deny that Coleridge had a mission with his poetry. As he once wrote in a letter, he was convinced that truth finds its best expression in poetry. A poet with such a conviction, a pastor’s son at that, a preacher, political orator and philosopher, must mean something with his poetry. Coleridge once even claimed that The Ancient Mariner was too preachy. It had 'too much moral', he said, it forced a devotional message on the reader too strongly.

Mirror of the soul
If we suspect that a poet has an intention with his poem, this does not necessarily mean that they themselves are aware of the intention of each of their lines. A poet can come up with a story as a broad sketch or symbolic treatment of a feeling or idea. But gradually images may arise –  spontaneously and unintentionally – from the deep wells of their thought, that emit others and combine to lead a life of their own. The story then runs away with the poet. The spontaneous images surprise the author, who can only determine afterwards whether they represent their intentions correctly or not. Or they are faced with a riddle: they cannot rationally interpret the product of their own imagination, and only feel meaning by intuition.  And there may be readers who, based on a completely different background than the poet, come to a plausible rational interpretation. Or readers who recognise their own, purely personal truths and feelings in the poem. The poet does not necessarily have to agree with all those interpretations. In that sense, poetry mirrors the reader's own soul.