Objection 1. It seems that there is no need for the repayment of gratitude to surpass the favor received. For it is not possible to make even equal repayment to some, for instance, one's parents, as the Philosopher states (Ethic. viii, 14). Now virtue does not attempt the impossible. Therefore gratitude for a favor does not tend to something yet greater.
Objection 2. Further, if one person repays another more than he has received by his favor, by that very fact he gives him something his turn, as it were. But the latter owes him repayment for the favor which in his turn the former has conferred on him. Therefore he that first conferred a favor will be bound to a yet greater repayment, and so on indefinitely. Now virtue does not strive at the indefinite, since "the indefinite removes the nature of good" (Metaph. ii, text. 8). Therefore repayment of gratitude should not surpass the favor received.
Objection 3. Further, justice consists in equality. But "more" is excess of equality. Since therefore excess is sinful in every virtue, it seems that to repay more than the favor received is sinful and opposed to justice.
On the contrary, The Philosopher says (Ethic. v, 5): "We should repay those who are gracious to us, by being gracious to them return," and this is done by repaying more than we have received. Therefore gratitude should incline to do something greater.
I answer that, As stated above (Article 5), gratitude regards the favor received according the intention of the benefactor; who seems be deserving of praise, chiefly for having conferred the favor gratis without being bound to do so. Wherefore the beneficiary is under a moral obligation to bestow something gratis in return. Now he does not seem to bestow something gratis, unless he exceeds the quantity of the favor received: because so long as he repays less or an equivalent, he would seem to do nothing gratis, but only to return what he has received. Therefore gratitude always inclines, as far as possible, to pay back something more.
Reply to Objection 1. As stated above (3, ad 5; 5), in repaying favors we must consider the disposition rather than the deed. Accordingly, if we consider the effect of beneficence, which a son receives from his parents namely, to be and to live, the son cannot make an equal repayment, as the Philosopher states (Ethic. viii, 14). But if we consider the will of the giver and of the repayer, then it is possible for the son to pay back something greater to his father, as Seneca declares (De Benef. ii.). If, however, he were unable to do so, the will to pay back would be sufficient for gratitude.
Reply to Objection 2. The debt of gratitude flows from charity, which the more it is paid the more it is due, according to Romans 13:8, "Owe no man anything, but to love one another." Wherefore it is not unreasonable if the obligation of gratitude has no limit.
Reply to Objection 3. As in injustice, which is a cardinal virtue, we consider equality of things, so in gratitude we consider equality of wills. For while on the one hand the benefactor of his own free-will gave something he was not bound to give, so on the other hand the beneficiary repays something over and above what he has received.
Source: Summa Theologica (New Advent)