Objection 1. It seems that ingratitude is not always a sin. For Seneca says (De Benef. ii.) that "he who does not repay a favor is ungrateful." But sometimes it is impossible to repay a favor without sinning, for instance if one man has helped another to commit a sin. Therefore, since it is not a sin to refrain from sinning, it seems that ingratitude is not always a sin.
Objection 2. Further, every sin is in the power of the person who commits it: because, according to Augustine (De Lib. Arb. iii; Retract. i), "no man sins in what he cannot avoid." Now sometimes it is not in the power of the sinner to avoid ingratitude, for instance when he has not the means of repaying. Again forgetfulness is not in our power, and yet Seneca declares (De Benef. ii.) that "to forget a kindness is the height of ingratitude." Therefore ingratitude is not always a sin.
Objection 3. Further, there would seem to be no repayment in being unwilling to owe anything, according to the Apostle (Romans 13:8), "Owe no man anything." Yet "an unwilling debtor is ungrateful," as Seneca declares (De Benef. iv). Therefore ingratitude is not always a sin.
On the contrary, Ingratitude is reckoned among other sins (2 Timothy 3:2), where it is written: "Disobedient to parents, ungrateful, wicked." etc.
I answer that, As stated above (106, 4, ad 1, 6) a debt of gratitude is a moral debt required by virtue. Now a thing is a sin from the fact of its being contrary to virtue. Wherefore it is evident that every ingratitude is a sin.
Reply to Objection 1. Gratitude regards a favor received: and he that helps another to commit a sin does him not a favor but an injury: and so no thanks are due to him, except perhaps on account of his good will, supposing him to have been deceived, and to have thought to help him in doing good, whereas he helped him to sin. On such a case the repayment due to him is not that he should be helped to commit a sin, because this would be repaying not good but evil, and this is contrary to gratitude.
Reply to Objection 2. No man is excused from ingratitude through inability to repay, for the very reason that the mere will suffices for the repayment of the debt of gratitude, as stated above (106, 6, ad 1).
Forgetfulness of a favor received amounts to ingratitude, not indeed the forgetfulness that arises from a natural defect, that is not subject to the will, but that which arises from negligence. For, as Seneca observes (De Benef. ii.), "when forgetfulness of favors lays hold of a man, he has apparently given little thought to their repayment."
Reply to Objection 3. The debt of gratitude flows from the debt of love, and from the latter no man should wish to be free. Hence that anyone should owe this debt unwillingly seems to arise from lack of love for his benefactor.
Source: Summa Theologica (New Advent)