Some characters have a particular trait or mannerism that's come to be viewed as an overall part of them. Maybe they're stingy, or abrasive, or just like using a lot of profanity. Along comes an episode with An Aesop, and a character learns how good it is to be generous, or friendly, or that they don't need curse words to make themselves known. Then, by two episodes later at the most, they're back to hoarding their money, snapping at people, or cursing like a sailor. They've just run into Aesop Amnesia. And, of course, it allows the character to learn the same lesson all over again later!