The lesson to be learned from Eternal Sunshine is that history repeats itself. Especially if you don't learn from it.
So if a person has no associated memories with something terrible, then they may begin to think that the terrible thing is not so terrible. Also, they would never be able to learn from the first situation.
Slashdot Mirror
And by pretending it never happened, the same pattern occurred because the person had erased the lesson from the first time.
The lesson to be learned from Eternal Sunshine is that history repeats itself. Especially if you don't learn from it. So if a person has no associated memories with something terrible, then they may begin to think that the terrible thing is not so terrible. Also, they would never be able to learn from the first situation.