In Dante's Christian universe, every motion – from the petty choices of humans to the cosmic revolutions of the stars – is motivated by love. Thus love for God gives the entire universe its proper order, for everything moves in accord with God's will. Appropriately, then, God's love is represented as light, which grows ever brighter the closer to Heaven one gets. Charity is the best and most selfless kind of love, most sanctioned by God. Finally, providence is a special brand of divine mercy or compassion that allows for seeming deviations in God's plan but ultimately has mankind's well-being at heart.
The primary difference between the sinners in Hell and the blessed in Heaven is that the blessed have learned to love the correct object and in proper measure, while sinners simply have not.
The same principle that guides the blessed to their proper places in Heaven governs the seemingly unjust punishments of sinners in Hell; in short, love is the basis of justice.