1....Behold the Psalm sounds; it is the voice of some one (and that some one are you, if you will), of some one encouraging his soul to praise God, and saying to himself, “Praise the Lord, O my soul”. For sometimes in the tribulations and temptations of this present life, whether we will or no, our soul is troubled; of which troubling he speaks in another Psalm. But to remove this troubling, he suggests joy; not as yet in reality, but in hope; and says to it when troubled and anxious, sad and sorrowing, “Hope in God, for I will yet confess to Him.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)