Nami literally is Luffy's wife. She nags at him. Restrains him. Attacks him when he's an idiot. Comforts him. Stands by him. Defends him. Navigates for him.
Nami was introduced in the arc where Luffy said the Straw Hat was his greatest treasure. In Arlong Park, Luffy, placed the Straw Hat on Nami's head, even after she had done nothing but reject him and tell him to leave. Even after being betrayed by Nami, Luffy trusted her enough to give her his greatest treasure.
Nami is Luffy's compass/anchor. Nami's the one who make sure Luffy gets where he's going, and, just as Luffy is a natural parent towards kids, so is Nami.
People are used to romance being conveyed through sex and flirting. Nami and Luffy's love is shown through how they stand by each other and support each other, and that's why I LOVE their relationship. It's not about desire at all. Luffy loves Nami because she's his Nakama, and Nami loves Luffy because he saved the place and people she loved most. Their love is so pure. I love Luffy x Nami so much.