One main theme in Rime of the Ancient Mariner concerns the consequences of a single unthinking act. Like other Romantics, Coleridge believed that the seeds of destruction and creation are contained each within the other. A person cannot create something without destroying something else and likewise the destruction of one thing leads to the creation of something new. For example, the loss of the mariner's ship, shipmates and his former self lead to the regeneration of the mariner.

A second main theme is as the mariner begins to realize the consequences of his act and he struggles to accept responsibility for it, he must figure out that all things in nature are equal--that everything, as a part of nature, has its own beauty and should be cherished. It becomes apparent that the mariner realizes this when he recognizes the beauty of the sea snakes and his heart fills with love for them and he blesses them. [1]


Sources:
[1] http://uk.encarta.msn.com/sidebar_701610967/%E2%80%9CRime_of_the_Ancient_Mariner_The%E2%80%9D.html