I don't know why people are comparing Zoro's sacrifice and Sanji's desires to repay Zeff. They are two different stages in their development. Zoro's character flaw was his single mindedness, with Luffy but a means to an end. His sacrifice for Luffy was the end result of his development. Sanji felt a sense of gratitude to Zeff. He couldn't leave Zeff as he viewed that a selfish endeavor. Seeing how far Luffy and Zoro would go for their ambitions at the time was only there to remind him there's something admirable to the romanticism. And Zeff could see Luffy reignited the fire Sanji tried to repress.