16 Nevertheless, let us not overlook the fact that it is something spiritual that the Lord was promising. What means, “Whoso shall drink of this water shall thirst again?” It is true as to this water; it is true as to what the water signified. Since the water in the well is the pleasure of the world in its dark depth: from this men draw it with the vessel of lusts. Stooping forward, they let down the lust to reach the pleasure fetched from the depth of the well, and enjoy the pleasure and the preceding lust let down to fetch it.
For he who has not dispatched his lust in advance cannot get to the pleasure. Consider lust, then, as the vessel; and pleasure as the water from the depth of the well: when one has got at the pleasure of this world, it is meat to him, it is drink, it is a bath, a show, an amour; can it be that he will not thirst again? Therefore, “Whoso shall drink of this water,” says He, “will thirst again;” but if he shall receive water of me, “he shall never thirst.” “We shall be satisfied,” it says, “with the good things of Your house.” Of what water, then, is He to give, but of that of which it is said, “With You is the fountain of life”? For how shall they thirst, who “shall be drunk with the fatness of Your house”?
Source: Tractates on the Gospel of John (New Advent)