Sanji is pretty obviously shown as Straw Hat protagonist material by his very nature. He is passionate, has a sense of personal honor, and is shown as an outcast from the rest of the kitchen staff. Even though the cooks have a reputation of violence, they judge Sanji as a problem for being violent. From Sanji’s perspective, he is just standing up for the kitchen staff and the integrity of the food, and that attachment to success is what attracts Luffy to invite him to the crew.
Sanji has some sort of emotional ties to working on the restaurant ship, and it makes sense that it would take something “emotional” such as falling “in love” with Nami to break his obsession with working for the old man, but I hope they don’t play this up a creepy angle. While there is some value, narratively & comedicaly, in the unrequited love cliche, it is quickly overdone.
Last note, Luffy appears to be taking the attitude of doing a task so poorly, he is not asked to do it again. I learned this technique from Calvin & Hobbes and it is true. People much prefer important actions to be done by people with an attention to detail and an interest in positive results. Hiring Luffy under duress ensures that the tasks will not be done perfectly. A workforce needs to feel valued & appreciated if they are going to work to the best of their abilities.