Thomas Aquinas, Compendium of Theology

This enables us to understand that the soul which sees God--and the same is true of any other spiritual creature--has its will firmly fixed in Him, so that it can never turn to what is opposed to Him. For, since the object of the will is the good, the will cannot incline to anything whatever unless it exhibits some aspect of good. Any particular good may be wanting in some perfection, which the knower is then free to seek in another quarter. Therefore the will of him who beholds some particular good need not rest content with its possession, but may search farther afield beyond its orbit. But God, who is universal good and very goodness itself, is not lacking in any good that may be sought elsewhere, as was shown above. And so those who enjoy the vision of God's essence cannot turn their will from Him, but must rather desire all things in subordination to Him.

Something similar is observed in the process of understanding. Our mind, when in doubt, can turn this way and that, until it reaches a first principle; then the intellect must come to a halt. Since the end has the same function in the field of desire that a principle has in the matter of understanding, the will can veer in opposite directions until it comes to the knowledge or fruition of the last end, in which it must rest. The nature of perfect happiness would be contradicted if man, after achieving it, could turn to what is opposed to it. For then fear of losing happiness would not be wholly excluded, and so desire would not be completely satisfied. Hence the Apocalypse 3:12 says of the blessed person, that "he shall go out no more."