6 “Make glad the soul of Your servant: for unto You, O Lord, have I lifted up my soul”. Make it glad, for unto You have I lifted it up. For it was on earth, and from the earth it felt bitterness: lest it should wither away in bitterness, lest it should lose all the sweetness of Your grace, I lifted it up unto You: make it glad with Yourself. For You alone are gladness: the whole world is full of bitterness. Surely with reason He admonishes His members to lift up their hearts.
May they hear and do it: may they lift up unto Him what on earth is ill. There the heart decays not, if it be lifted up to God. If you had grain in your rooms below, you would take it up higher, lest it should grow rotten. Would you remove your grain, and do you suffer your heart to rot on the earth? You would take your grain up higher: lift up your heart to heaven. And how can I, do you say? What ropes are needed? What machines? What ladders? Your affections are the steps: your will the way. By loving you mount, by neglect you descend. Standing on the earth you are in heaven, if you love God. For the heart is not so raised as the body is raised: the body to be lifted up changes its place: the heart to be lifted up changes its will.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)