Metaphysics of the Music, part 24
If God’s own goodness leads him to desire and to effect the existence of things other than himself, the same must invariably hold true for those rational beings whom he has made especially after his own image. As Thomas puts it in his Summa Contra Gentiles,
a thing approaches to God’s likeness the more perfectly as it resembles Him in more things. Now, goodness is in God, and the outpouring of goodness into other things. Hence, the creature approaches more perfectly to God’s likeness if it is not only good, but can also act for the good of other things, than if it were good only in itself; that which both shines and casts light is more like the sun than that which only shines. But no creature could act for the benefit of another creature unless plurality and inequality existed in created things. For the agent is distinct from the patient and superior to it. In order that there might be in created things a perfect representation of God, the existence of diverse grades among them was therefore necessary.
God creates other things to communicate his own goodness, but part of that goodness which he gives to other things is precisely his own propensity for bestowing goodness on others. Thus, in order for creatures to receive God’s goodness, they themselves must have things other than themselves onto whom they in their turn, yet in imitation of God, might pass on this goodness. The fulfillment of the nature of created things, therefore, necessitates the existence of things other than themselves towards whom they might manifest their (and their Creator’s) benevolence. Again, and as Thomas’s great Florentine student Dante well recognized, creation constitutes not a metaphysical tragedy, but a veritable “divine comedy”:
the greater the proportion of our love,
the more eternal goodness we receive;
the more souls there above who are in love
the more there are worth loving; love grows more,
each soul a mirror mutually mirroring.
 Summa Contra Gentiles 2.45, trans. Anderson. “Quanto aliquid in pluribus est Deo simile, tanto perfectius ad eius similitudinem accedit . In Deo autem est bonitas, et diffusio bonitatis in alia. Perfectius igitur accedit res creata ad Dei similitudinem si non solum bona est sed etiam ad bonitatem aliorum agere potest, quam si solum in se bona esset: sicut similius est soli quod lucet et illuminat quam quod lucet tantum. Non autem posset creatura ad bonitatem alterius creaturae agere nisi esset in rebus creatis pluralitas et inaequalitas: quia agens est aliud a patiente, et honorabilius eo. Oportuit igitur, ad hoc quod in creaturis esset perfecta Dei imitatio, quod diversi gradus in creaturis invenirentur.”
 Dante, The Divine Comedy: Purgatorio 15.71-5, trans. Musa.