What is the main theme in Samuel Taylor Coleridge's "The Rime of the Ancient Mariner?" What is the poem's ultimate goal?

The main theme of "The Rime of the Ancient Mariner" is expressed by the mariner just before he lets the wedding guests go. He states,

He prayeth well, who loveth well

Both man and bird and beast. 

He prayeth best, who loveth best

All things both great and small.

In other words, the person who will receive favor from God is the one who displays love towards all of God's creatures, including people, but...

The main theme of "The Rime of the Ancient Mariner" is expressed by the mariner just before he lets the wedding guests go. He states,

He prayeth well, who loveth well

Both man and bird and beast. 

He prayeth best, who loveth best

All things both great and small.

In other words, the person who will receive favor from God is the one who displays love towards all of God's creatures, including people, but also including animals and even the least admired members of creation. The theme is worked out in the plot of the poem when the mariner shoots an albatross that has been a friend to the ship's crew. Because of that cruel and senseless act, which the men at one point approve of, the crew (except the mariner) is killed and the mariner must experience "Life-in-Death." Only when the mariner looks with admiration on the water-snakes is he rewarded. His hard heart is able to pray at last, rain comes, and the polar spirits, with the help of seraph-men who inhabit the corpses, sail the ship back to harbor. The mariner must spend the rest of his life sharing his tale with people who need to hear it.

That is the theme of the poem. The goal, however, is certainly very different. One cannot help but be disappointed at the end of this harrowing tale to find this trite and somewhat shallow sentiment. Even Coleridge felt that he had been too heavy-handed in expressing the moral at the end of the poem, suggesting that the communication of the moral was not his "goal" in writing the poem. No doubt Coleridge's goal was the goal of all great poetry, namely, "the spontaneous overflow of powerful feelings." The poet, according to Wordsworth, feels deep passions and puts those feelings into beautiful, rhythmic, lyrical words in order to allow the reader to experience those same emotions. In Coleridge's poem, the harrowing tale of the supernatural--a tale of nail-biting Gothic enchantment--arouses fear, awe, disgust, anger, joy, and relief in the reader, making it a thrilling experience. Readers have enjoyed reading about the mariner's agonies for over two hundred years, attesting to the fact that Coleridge absolutely achieved his goal.