"... Early in the prayer, the psalmist recalled mentally the words of the arrogant adversaries (verse 3), but now, as his thoughts progress, it is the words of the pusillanimous doubters which come to mind, the people who say: “Who will show us good?” And the dubious help of doubters can sometimes be as dangerous as the arrogant words of enemies! An attack by enemies can be thwarted if the innocent person has trust in God, but when nagging doubts arise, confidence can easily be undermined. Enemies are real and all too visible, but is there anybody who can really help against them? Can God really do any good? And so the psalmist, as his confidence grows, must not only transcend the attacks of opponents, but must also overcome the voice of doubt within himself. And he overcomes it by recalling an ancient blessing, the Blessing of Aaron, which he had doubtless heard hundreds of times during his participation in worship: “The Lord will make his face to shine upon you and be gracious to you; the Lord will lift up his face to you and give you his peace” (Numbers 6:25–26). In a shortened form, he makes that blessing his own to quash the uncertainties within him. He prays for an inner joy which would be greater than those outward manifestations of joy which characterized his society, the joy and rejoicing which stemmed from a successful harvest of corn and grapes for new wine ..."

Cragie, Peter C. and Marvin E. Tate Word Biblical Commentary: Psalms 1-50 (p. 143) Zondervan, 2004