"The reason for this is that before temporal things are possessed, they are highly regarded and thought satisfying; but after they are possessed, they are found to be neither so great as thought nor sufficient to satisfy our desires, and so our desires are not satisfied but move on to something else. On the other hand, a spiritual thing is not known unless it is possessed: 'No one knows but he who receives it' (Rv 2:17). So, when it is not possessed, it does not produce a desire; but once it is possessed and known, then it brings pleasure and produces desire, but not to possess something else. Yet, because it is imperfectly known on account of the deficiency of the one receiving it, it produces a desire in us to possess it perfectly. We read of this thirst: 'My soul thirsted for God, the living fountain' (Ps 41:2).
"This thirst is not completely taken away in this world because in this life we cannot understand spiritual things; consequently, one who drinks this water will still thirst for its completion. But he will not always be thirsty, as though the water will run out, for we read (Ps 35:9): 'They will be intoxicated from the richness of your house.' In the life of glory, where the blessed drink perfectly the water of divine grace, they will never be thirsty again: 'Blessed are they who hunger and thirst for what is right,' that is, in this world, 'for they will be satisfied,' in the life of glory (Mt 5:6)."
—St. Thomas Aquinas, Commentary on the Gospel of St. John, chapter 4, lecture 2 (par. 586).
