Isn’t it astonishing just how much love and respect Sanji has for Zeff?
Not using his hands while fighting or never kicking women is one thing, but saving his abusers because of his concerns about what his father would think takes Sanji’s devotion to Zeff to an entirely different level. His kind heart did play a part yes, but more than anything he wants to be a son who lives up to his father’s expectations and make him proud, not because he is craving for acknowledgement but because he truly, deeply loves him. Zeff would probably never figure out what truly transpired, but regardless of that Sanji refused to be a disappointment and risked many many things, including his nakama’s safety. Maybe that is why Zeff has faith in Sanji that he will always be the son he can be proud of, no matter where he is or what he is doing.