The thing with Luffy is that Sanji knows he's incapable of lying. His captain only says what he believes to be the truth, and he doesn't bother with 'complicated'. The man told him himself, once. When he praises or reassures him, Sanji knows Luffy means it. It makes him feel naked in a different way—raw and vulnerable. Loved and cherished too, which is the most surprising part. 

"Thank you, Luffy." Sanji's voice shakes a little with emotion, and it's embarrassing, but he hopes that Luffy can read in those simple 3 words all he means. How much the words got to him. To drive the point home, he gently works on Luffy's hair and smiles once the other man looks at him.

"I'm lucky to have you. And I want to take care of you too..." He's already made Luffy hot and bothered before, he might as well finish what he started.  The hand that was at Luffy's waist slides down over the curve of his hip, palm caressing under his lower belly and fingers playing with the man's happy trail before they brush against the base of his cock. "If you'd let me?"