OnePiece

In SBS he once explained that these hits between friends or family have a lot of love imbued in them, which renders things like Luffy's rubber useless for defense. It doesn't work on enemies because there's no mutual love between Nami and her opponents.

~~That's just an excuse though, the real answer is it's cartoon logic~~

Edit: references: https://anime.stackexchange.com/questions/8001/how-can-namis-punches-hurt-luffy-so-much