Commentary Purg XXX 142-145

As Beatrice wept in Virgil's presence in hell for Dante's sake (verse 141), so now it is Dante's turn to weep for the sins that made her intervention necessary.  The angels may want to celebrate the eventual triumph of this saved Christian; Beatrice is here to make sure that he observes the ritual of the completion of purgation correctly, even on this trial run.