hmmm...that's an interesting quesiton because there are many different interpertations of this poem.
The main thing that captures the speakers interest in the urn, is the idea of time and art and beauty. The lovers on the urn are frozen in time, for all eternity. Although the "bold lover" will never catch the girl, the girl will never age. The urn is beautiful to the speaker because of the fact that it will never change for all eternity. The beauty also comes from a safe place. One way of looking at the "truth and beauty" statement is to consider that the scene on the urn is true and beautiful because it is self-contained: it has no need for answers, and so it will always have found its truth, unlike real life, where new details always rise up and make truth and beauty elusive. The common factor to both truth and beauty in this poem is that they both occur when you know all that you need to know, regardless of what is happening around you.