The concept of love Dante presents in the Purgatorio has a theological bent. Love ultimately comes from God, who is Infinite Love and instills it in each of his creatures. However, being a loving God, He allows each man free will by dividing up man's loves (desires) into natural and mental; the natural inherently loves the ultimate good (God), while the mental love can desire whatever attracts it (usually beautiful things) and must be trained to desire only worthy things. All of the sins punished in Purgatory are forms of perverted love or love expressed in improper measure. Perhaps the most shocking idea, though, is that love motivates all human action.
Given Virgil’s definition of mental love, prayer is perhaps the most selfless – and therefore superior – kind of mental love, one in which a soul prays for another, whom he may or may not know, out of the pure goodness of his heart.
In Purgatorio, Dante’s image of Beatrice conflates the concepts of mortal and divine love and it is clear that, as a mortal, he still has trouble differentiating the two.