How can Nami hurt Luffy so much?

D: Blows shouldn’t hurt Luffy, so why does he get bruises and swelling when Nami and the others beat the crap out of him?! Tell me!!!

Episode 215

 

Episode 316

 

Episode 572

 

O: When the crew beats up on each other, it’s always in admonishment with a strong sense of purpose, which is why it works so well!! It works on his spirit!! I bet Nami is thinking, “It’s his spirit that hurts deep down!!” …right? I guess not.