Gratitude calls not only for the formation of certain beliefs, but for their persistence as well. A beneficiary who forgets an act of beneficence (or, at least, forgets too quickly) seems to fall short of what gratitude calls for (Aquinas II–II, q. 104, art. 1; Seneca On Benefits: II.10.4; III.5.2).
In fact, according to Seneca, a beneficiary who forgets a benefit is “the worst and most ungrateful” of beneficiaries—perhaps because merely remembering a benefit is the easiest part of being grateful (Seneca On Benefits: III.5.2).
Gratitude Apprentice 