a nice exposition of Psalm 42 came up, by Ray C. Stedman. The whole page is worth reading, but his dealing with verse 7 resonates with my intuitive understanding of what the Psalmist is getting at.
He is still despondent. His remembering of the past has not worked. Usually it does. Usually this is enough to deliver us from this nagging fear that God is not going to do anything at all. But now it does not, and so he tries to help by remembering something else: an experience that he had when he was in the northern part of Israel near Mount Hermon, at the head of the Jordan River, on a little peak of the range where Mount Hermon is located, called Mount Mizar (which, incidentally, means “little mountain”). On that occasion he could hear the waterfalls of that mountainous region, the thundering cataracts. He became aware of how they seemed to be calling to one another, “deep calling unto deep,” and it reminded him that the deeps in God call out to the deeps in man.
One of the amazing things about nature is the silent voices that call to one another across vast spaces. The moon calls to the deeps in the sea, raising the tides. Twice a day the waters rise in tides across the earth, because of the moon calling to the ocean. You know how the sun and the rain call to the deeps in a seed, causing it to stir with life and to spring up and grow. There are vast distances that call to the deeps in wild birds, causing them to wing their way across trackless wastes to lay their eggs; there are voices that call to certain fish, sending them across the seas to spawn. In this way the Psalmist is reminded that God also calls to man. There are deeps in God that correspond with deeps in man, and he calls to them. The Psalmist specifically names two here: the deeps of the love of God, and the joy of God, calling out to the corresponding deeps of prayer in the believer.