Purgatorio ends on perhaps one of the best cliffhangers ever. Having grown so much spiritually during his journey through all seven terraces and finally receiving the final cleansing at the hands of Matilda, Dante is poised for his ultimate reward: to ascend to Heaven with Beatrice at his side. Just as he's about to ascend, the book... ends. Arrrrgh.
Now that we have vented our frustration, we can look at the ending more objectively and see how it does actually provide a sense of closure and sets up a transition to the last part of the Divine Comedy, Paradiso.
First of all, this is the farewell for Virgil. As a pagan poet, he no longer has the right to guide Dante through realms that increasingly require faith over human reason. His status as a virtuous pagan, a renowned epic poet, and Dante’s idol will no longer cut it.
He must step down and relinquish his position to Statius, a poet who has all the qualities Virgil has: well-deserved fame and epic works. On top of that, he also has Christian faith. He is introduced in Canto XXI and from that point his influence only grows, while Virgil’s wanes. Although he never explicitly states why he concedes discussions about philosophy to Statius, Virgil has hinted before that the limits of his knowledge are being tested and that, although his capacity to reason is as keen as ever, the territory Dante is venturing into requires more than just human reason.
So Virgil's increasing reticence from Canto XXI onwards and his absolute silence in the Earthly Paradise prepare us for his eventual departure. By the time Dante turns to find him gone, Virgil has already been a virtual non-presence in the text for some time. His disappearance elicits some tears from Dante, but it also heralds a new beginning, where paganism has no place and where human reason becomes increasingly obsolete as Dante begins to engage us with more fantastic discourse based more on theology than philosophy.
The ending at the same time ties up and puts a twist on a number of our themes. Dante's conception of love has developed significantly since his visit to Hell. We now understand why love is not always inherently good, how it acts as the motive for every human action, and even how it can lead to sin and damnation. We even have an inkling of how love might create a place as hellish as… well… Hell.
But the concept of love really hits home when Dante, upon leaving Purgatory proper, is deemed perfect in his mental love, having aligned it with God. For Dante, this transition represents the opening up of a whole new realm of possibility; for Virgil, it means the end of his mentorship of Dante. As for Statius, he now has a companion with whom to traverse Heaven.
Structurally, the ending brings the second part of Dante’s education to the close, perhaps the most practical part of his spiritual education. Certainly, it completes Dante’s last vestiges of suffering, for having endured Hell and purged himself of his sins, Dante has now only to enjoy his reward. But perhaps most importantly, the ending of Purgatorio establishes Beatrice as a central character for the ongoing narrative in Paradiso.
As a symbol of Divine Knowledge and Justice, she brings Dante unscathed through the harshest, most personal part of his penance. Now, with the love story between them somewhat unresolved and Dante’s acquisition of Divine Knowledge still incomplete, she looks to be a central presence in his experiences in Paradiso.