7 “Serve the Lord with gladness.” All servitude is full of bitterness: all who are bound to a lot of servitude both are slaves, and discontented. Fear not the servitude of that Lord: there will be no groaning there, no discontent, no indignation; no one seeks to be sold to another master, since it is a sweet service, because we are all redeemed. Great happiness, brethren, it is, to be a slave in that great house, although in bonds. Fear not, bound slave, confess unto the Lord: ascribe your bonds to your own deservings; confess in your chains, if you are desirous they be changed into ornaments....At the same time you are slave, and free; slave, because you are created such; free, because you are loved by God, by whom you were created: yea, free indeed, because you love Him by whom you were made. Serve not with discontent; for your murmurs do not tend to release you from serving, but to make you a wicked servant. You are a slave of the Lord, you are a freedman of the Lord: seek not so to be emancipated as to depart from the house of Him who frees you....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)