Angels can fly because they take themselves lightly.

A bird is active, because a bird is soft. A stone is helpless, because a stone is hard. A stone must by its own nature go downwards, because hardness is weakness. The bird can of its nature go upwards, because fragility is force. In perfect force there is a kind of frivolity, an airiness that can maintain itself in the air.... Angels can fly because they take themselves lightly.

This his always been the instinct of Christendom.... [P]ride cannot rise to levity and levitation. Pride is the downward drag of all things into an easy solemnity. One 'settles down' into a sort of selfish seriousness; but one has to rise to a gay self-forgetfulness. A man 'falls' into a brown study; he reaches up at a blue sky. Seriousness is not a virtue.... It is really a natural trend or lapse into taking one's self gravely, because it is the easiest thing to do. It is much easier to write a good Times leading article than a good joke in Punch. For solemnity flows out of men naturally; but laughter is a leap. It is easy to be heavy: hard to be light. Satan fell by the force of gravity.

—G.K. Chesterton, Orthodoxy (1908)