Anre marapadhu nanru
To forget a good deed is just 'not done', whereas an ill-turn is best forgotten then and there.
It is not at all the right thing to forget acts of kindness. It is just 'not once', as the Englishman would say. But any injury, on the contrary, is best forgotten immediately.
It is the hallmark of the good ones of the earth, to remember only the good acts done and forget all the rest immediately. The great Confucius of China himself once said 'Forget injuries, never forget kindnesses.'
A corresponding injuction is found in 'Athichudi.'