As it's been so long without an answer, I will put in my non-native 5 cents. I also put "spoil" and it was not accepted. I think the reason will be the idea that "memper" verbs increase a state. So the English needs to imply that the mood wasn't good in the first place.
'memper-' ==> 'the state before was already like the [base word], now the [base word] is there in a higher degree'.