"Luffy is the man that convinced her that her life was even still worth living."
You make it sound so intelligent. All he did was allow her on his crew in a scene that was played as a joke and insincerely.
Robin is an intelligent character. Which of the following two scenerios d'you think would be more likely to convince an inteligent character to change their self-image
a) a logical conversation with Franky, like the one they have on the train ride.
b) an ADHD teenage pirate, screaming at the top of his lungs like a brat who wants his lollipop. And he only wants her back for himself, because loosing a crew member for him is like having one of your toys taken away.
That is the mindset that was created for him. Luffy's biggest problem has always been, that he only ever seems intelligent or sincere when he is violent. It's as if Oda wants to say that being a ruffian is what gets you places.
(now considering who is about to enter the oval office, I suppose that is sometimes true)