Objection 1. It would seem that it is more praiseworthy and meritorious to do a thing without a vow than in fulfilment of a vow. Prosper says (De Vita Contempl. ii): "We should abstain or fast without putting ourselves under the necessity of fasting, lest that which we are free to do be done without devotion and unwillingly." Now he who vows to fast puts himself under the necessity of fasting. Therefore it would be better for him to fast without taking the vow.
Objection 2. Further, the Apostle says (2 Corinthians 9:7): "Everyone as he hath determined in his heart, not with sadness, or of necessity: for God loveth a cheerful giver." Now some fulfil sorrowfully what they have vowed: and this seems to be due to the necessity arising from the vow, for necessity is a cause of sorrow according to Metaph. v [Ed. Did. iv, 5]. Therefore, it is better to do something without a vow, than in fulfilment of a vow.
Objection 3. Further, a vow is necessary for the purpose of fixing the will on that which is vowed, as stated above (Article 4). But the will cannot be more fixed on a thing than when it actually does that thing. Therefore it is no better to do a thing in fulfilment of a vow than without a vow.
On the contrary, A gloss on the words of Psalm 75:12, "Vow ye and pay," says: "Vows are counseled to the will." But a counsel is about none but a better good. Therefore it is better to do a deed in fulfilment of a vow than without a vow: since he that does it without a vow fulfils only one counsel, viz. the counsel to do it, whereas he that does it with a vow, fulfils two counsels, viz. the counsel to vow and the counsel to do it.
I answer that, For three reasons it is better and more meritorious to do one and the same deed with a vow than without. First, because to vow, as stated above (Article 5) is an act of religion which is the chief of the moral virtues. Now the more excellent the virtue the better and more meritorious the deed. Wherefore the act of an inferior virtue is the better the more meritorious for being commanded by a superior virtue, whose act it becomes through being commanded by it, just as the act of faith or hope is better if it be commanded by charity. Hence the works of the other moral virtues (for instance, fasting, which is an act of abstinence; and being continent, which is an act of chastity) are better and more meritorious, if they be done in fulfilment of a vow, since thus they belong to the divine worship, being like sacrifices to God. Wherefore Augustine says (De Virg. viii) that "not even is virginity honorable as such, but only when it is consecrated to God, and cherished by godly continence."
Secondly, because he that vows something and does it, subjects himself to God more than he that only does it; for he subjects himself to God not only as to the act, but also as to the power, since in future he cannot do something else. Even so he gives more who gives the tree with its fruit, than he that gives the fruit only, as Anselm [Eadmer] observes (De Simil. viii). For this reason, we thank even those who promise, as stated above (5, ad 2).
Thirdly, because a vow fixes the will on the good immovably and to do anything of a will that is fixed on the good belongs to the perfection of virtue, according to the Philosopher (Ethic. ii, 4), just as to sin with an obstinate mind aggravates the sin, and is called a sin against the Holy Ghost, as stated above (Question 14, Article 2).
Reply to Objection 1. The passage quoted should be understood as referring to necessity of coercion which causes an act to be involuntary and excludes devotion. Hence he says pointedly: "Lest that which we are free to do be done without devotion and unwillingly." On the other hand the necessity resulting from a vow is caused by the immobility of the will, wherefore it strengthens the will and increases devotion. Hence the argument does not conclude.
Reply to Objection 2. According to the Philosopher, necessity of coercion, in so far as it is opposed to the will, causes sorrow. But the necessity resulting from a vow, in those who are well disposed, in so far as it strengthens the will, causes not sorrow but joy. Hence Augustine says (Ep. ad Arment. et Paulin. cxxcii): "Repent not of thy vow: thou shouldst rather rejoice that thou canst no longer do what thou mightest lawfully have done to thy detriment." If, however, the very deed, considered in itself, were to become disagreeable and involuntary after one has taken the vow, the will to fulfil it remaining withal, it is still more meritorious than if it were done without the vow, since the fulfilment of a vow is an act of religion which is a greater virtue than abstinence, of which fasting is an act.
Reply to Objection 3. He who does something without having vowed it has an immovable will as regards the individual deed which he does and at the time when he does it; but his will does not remain altogether fixed for the time to come, as does the will of one who makes a vow: for the latter has bound his will to do something, both before he did that particular deed, and perchance to do it many times.
Source: Summa Theologica (New Advent)