Sanji was head over heels in love with Nami from the moment he saw her in the Baratie, and his infatuation for the navigator hasn’t wavered once during his whole time spent travelling with her (despite meeting other beautiful women on their voyage).
Nami, on her part, was pleased she was getting special attention, leading her to manipulate the cook numerous times. Sanji appears to be well aware of this manipulation, but is happy to continue being constantly manipulated; in fact, he claimed that Nami was 98.72% of the reason he joined Luffy’s crew in the first place. Sanji hates treating Nami roughly in any manner and is always gentle with her, this is another reason why Sanji fights with Zoro. Her safety is a frequent cause of his own injuries.
Despite her occasional annoyance at his antics, Nami does respect Sanji to some extent although she does not always show it. She is always shocked and distressed whenever Sanji loses a fight and is even prepared to rush to his aid in some battles.
Despite Sanji’s lecherous nature, Nami still cares and has concern for Sanji though usually she refrains from showing it in front of him (knowing what his reaction would be). This care was shown once in Zou when Sanji was unable to return to the crew, Nami cried to Luffy, over Sanji’s sacrifice and was the most distraught crew member over his absence, despite the others reassuring her that Sanji would be fine.
When Nami saw Sanji again, she was happy to see him again, but their excitement was short-lived as Sanji (reluctantly) tried to get them to leave Whole Cake Island by kicking Luffy away and acted as if he embraced his royal heritage. Nami was speechless and heart broken at Sanji’s supposed change in personality as he went on to insult them because of their peasant status. She was shocked to see Sanji attack a non-retaliating Luffy while insulting the crew and their dreams, and was even scared when he gave her a vicious glare to silence her (something he never did before).
After the confrontation, a tearful Nami approached Sanji and slapped him across the face while not using her usual “-kun” honorific for him, signifying a loss of respect.
She angrily and sarcastically bid him farewell on his journey while Sanji is unable to look her in the eye, instead simply walking away after the strike to his cheek. As Sanji rode away in the carriage, Nami cried in earnest and even attempted to get Luffy to abandon trying to rescue Sanji. When his crewmates were captured by Big Mom’s forces, Sanji believed he saved their life by agreeing to marry Pudding.
After coming to understand the situation, Nami was deeply saddened by Sanji’s troubles and once again becomes determined to rescue him and bring him back to the crew at all costs. While she does tell him that she’ll never forgive him for the despair he put her through, she’s willing to put that aside for the sake of rescuing him.
Despite her promise of never forgiving him, she felt relieved when he saved her from hitting the ground after his sister freed her from Smoothie’s grasp.