Jesus was not using terms that were bound to be misunderstood; He was using terms that anyone with spiritual insight should have understood. The promise was that the chosen people would draw water with joy from the wells of salvation (Isaiah 12:3). The psalmist spoke of his soul being thirsty for the living God (Psalm 42:1). God's promise was: "I will pour water on the thirsty land" (Isaiah 44:3). The summons was that everyone who was thirsty should come to the waters and freely drink (Isaiah 55:1).
Sometimes the Rabbis identified this living water with the wisdom of the law; sometimes they identified it with nothing less than the Holy Spirit of God. All Jewish pictorial religious language was full of this idea of the thirst of the soul which could be quenched only with the living water which was the gift of God.
At the heart of this, there is the fundamental truth that in the human heart there is a thirst for something that only Jesus Christ can satisfy. We are desperately unhappy about something - and we don't know what it is. In each of us, there is this nameless unsatisfied longing, this vague discontent, this something lacking, this frustration.
In his novel Sorrell and Son, Warwick Deeping tells of a conversation between Sorrell and his son. The boy is talking about life. He says that it is like groping in an enchanted fog. The fog breaks for a moment; you see the moon or a girl's face; you think you want the moor or the face; and then the fog comes down again and leaves you groping for something, you don't know what. Augustine talks about 'our hearts being restless till they find rest in Thee.'
Part of the human situation is that we cannot find happiness out of the things that the human situation has to offer. We are never safe from the longing for eternity which God has put into the human soul. There is a thirst which only Jesus Christ can satisfy.