Perfection
The complete perfection of the universe demands that there should be created natures which return to God, not only according to the likeness of their being, but also through their actions. This can only be through the acts of the reason and the will, because God himself has no other way of acting towards himself. Hence, for the final perfection of the universe, it was necessary that there should be intellectual beings. (Aquinas C. G. 2, 6)