In the Rime of Ancient Mariner why is the wedding guest sad and wiser after hearing the story?

Imagine yourself at a wedding, gl of champagne in hand, pretty dress on, new shoes, pedicure....you are approached by a crumple-faced vet with filthy hands and bad breath. He grabs your arm and tells you his story. You overcome your initial repulsion and listen. His words take you places you've never been; through them you experience a modi of his loss and pain -- that is one element of the human experience, that by hearing a story, and feeling the storyteller's pain or joy vicariously, one's own perceptions of the world become more complicated; richer. The vet leaves, you wash your hands, sip your champagne, but cannot help feeling a bit sad and a bit wiser.