Gratitude

"I answer that, ...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."
~St. Thomas Aquinas