The people on this urn never have to deal with changes in their world. Their world is permanently frozen in a single moment. The poem contrasts the timeless world of the urn with the upside-down hourglass that is human life. In the real world, joyful panting after an erotic chase can easily turn into a fever and a bad case of dry mouth. What was once sweet can become a "cloying" mess.
The final lines of the poem suggest that all we can do as humans is cling to whatever momentary beauty we can find.
The speaker comes to take a more charitable view of transience by the end of the poem.