“The psalm falls into three parts, each closing with the same refrain. Longings and tears, remembrances of festal hours passed in the sanctuary melt the singer’s soul, while taunting enemies hiss continual sarcasms at him as forsaken by his God. But his truer self silences these lamentations, and cheers the feebler “soul” with clear notes of trust and hope, blown in the refrain, like some trumpet clang rallying dispirited fugitives to the fight. The stimulus serves for a moment; but once more courage fails, and once more, at yet greater length and with yet sadder tones, plaints and longings are wailed forth. Once more, too, the higher self repeats its half-rebuke, half-encouragement. So ends the first of the psalms; but obviously it is no real ending, for the victory over fear is not won, and longing has not become blessed. So once more the wave of emotion rolls over the psalmist, but with a new aspect which makes all the difference. He prays now; he had only remembered and complained and said that he would pray before. Therefore now he triumphs, and though he still is keenly conscious of his enemies, they appear but for a moment, and though he still feels that he is far from the sanctuary, his heart goes out in hopeful visions of the gladness of his return thither, and he already tastes the rapture of the joy that will then flood his heart. Therefore the refrain comes for a third time; and this time the longing, trembling soul continues at the height to which the better self has lifted it, and silently acknowledges that it need not have been cast down. Thus the whole song is a picture of a soul climbing, not without backward slips, from the depths to the heights, or, in another aspect, of the transformation of longing into certainty of fruition, which is itself fruition after a kind.
Perhaps the singer had seen, during his exile on the eastern side of Jordan, some gentle creature, with open mouth and heaving flanks, eagerly seeking in dry wadies for a drop of water to cool her outstretched tongue; and the sight had struck on his heart as an image of himself longing for the presence of God in the sanctuary. A similar bit of local colour is generally recognised in Psalm 42:7. Nature reflects the poet’s moods, and overmastering emotion sees its own analogues everywhere. That lovely metaphor has touched the common heart as few have done, and the solitary singer’s plaint has fitted all devout lips. Injustice is done it, if it is regarded merely as the longing of a Levite for approach to the sanctuary. No doubt the psalmist connected communion with God and presence in the Temple more closely together than they should do who have heard the great charter, “neither in this mountain, nor in Jerusalem”; but, however the two things were coupled in his mind, they were sufficiently separate to allow of approach by longing and prayer while distant in body, and the true object of yearning was not access to the Temple, but communion with the God of the Temple.”
LINK: http://biblehub.com/commentaries/expositors/psalms/42.htm