The wedding guest, in "The Rime of the Ancient Mariner," leaves the story told by the old man "sadder but wiser." The events which the old mariner told the wedding guest are not those which evoke feelings of elation at all. The tale of the old mariner is one filled with death, sorrow, and fear.
In the end, the fate and consequences of the old mariner are meant to serve as his punishment for killing the albatross which tailed his ship. Not thinking, the old mariner kills the albatross and the consequences are dire.
In his attempt to make good on his bad deed, the old mariner is forced to tell his tale of sorrow to all who will listen. A wedding guest, as detailed by the poem, is the "newest" receiver of his knowledge.
The reason the wedding guest is left feeling saddened and wise is because the tale is one which speaks of consequences for actions. The wedding guest realizes that one must consider all consequences for any actions before acting upon a decision. The tale also gives the wedding guest a feeling of sadness for what the old mariner had to face. This shows the sympathy the wedding guest has for both the old mariner and humanity. In the end, the wedding guest learns to think about consequences (wiser) and sadder because of all of the loss the old mariner had to face.