He WAS her hero. And he would ALWAYS be. When it seems like the world was crashing down, he was there. He loved her for all her imperfections, He loved her best when she was a mess. He loved her. When she was her own worst enemy. No matter what.
He was her sunshine. He was her everything.
And without him, she felt like there would be no her.
And whenever he tells her it will be okay, she believes him. Even if it's all falling apart. Nami thinks of herself as one word : Lucky.
She can't even picture herself without him. Without Luffy, there would be no Nami.