Gregory the Dialogist

Man is born to labour, and the bird to flying. For 'man is born to labour,' in that he, who is furnished with the gift of reason, bethinks himself that it is wholly impossible for him to pass through this season of his pilgrimage without sorrowing. Hence when Paul was recounting his woes to his disciples, he justly added, For yourselves know that we are appointed thereto. …

Morals on the Book of Job, Book VI ↗

Thomas Aquinas

From the fact that he said, "Nothing on earth happens without cause," it is really clear that everything has a natural disposition suited to its own proper operation, from which it is apparent that the natural dispositions of things are not without a cause, but happen for a determined end. So Eliphaz says, "but man is born to toil and the bird to fly." For just as the proper motion which the nature of a bird requires is that it fly, so the bird must have the instruments from its nature suitable for flying, namely wings and feathers. …

Commentary on Job ↗