10 “Voluntarily I will sacrifice to You”. Who can even understand this good thing of the heart, at another's speaking thereof, unless in himself he has tasted it? What is, “Voluntarily I will sacrifice to You”?... For what sacrifice here shall I take, brethren? Or what worthily shall I offer to the Lord for His mercy? Victims shall I seek from flock of sheep, ram shall I select, for any bull in the herds shall I look out, frankincense indeed from the land of the Sabæans shall I bring?
What shall I do? What offer; except that whereof He speaks, “Sacrifice of praise shall honour Me”? Wherefore then “voluntarily”? Because truly I love that which I praise. I praise God, and in the self-same praise I rejoice: in the praise of Himself I rejoice, at whom being praised, I blush not. For He is not praised in the same manner as by those who love the theatrical follies is praised either by a charioteer, or a hunter, or actor of any kind, and by their praisers, other praisers are invited, are exhorted, to shout together: and when all have shouted, ofttimes, if their favourite is overcome, they are all put to the blush.
Not so is our God: be He praised with the will, loved with charity: let it be gratuitous (or voluntary) that He is loved and that He is praised. What is “gratuitous”? Himself for the sake of Himself, not for the sake of something else. For if you praise God in order that He may give you something else, no longer freely do you love God. You would blush, if your wife for the sake of riches were to love you, and perchance if poverty should befall you, should begin to think of adultery.
Seeing that therefore you would be loved by your partner freely, will you for anything else love God? What reward are you to receive of God, O covetous man? Not earth for you, but Himself He keeps, who made heaven and earth. “Voluntarily I will sacrifice to You:” do it not of necessity. For if for the sake of anything else you praise God, out of necessity you praise. These things also which He has given, because of the Giver are good things. For He gives entirely, He gives these temporal things: and to certain men to their good, to certain men to their harm, after the height and depth of His judgments....“Voluntarily I will sacrifice to You.”
Wherefore “voluntarily”? Because gratis. What is gratis? “And I will confess to Your name, O Lord, for it is a good thing:” for nothing else, but because a “good thing” it is. Does he say, “I will confess to Your name, O Lord,” because You give me fruitful manors, because You give me gold and silver, because You give me extended riches, abundant money, most exalted dignity? Nay. But what? “For it is a good thing.” Nothing I find better than Your name.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)