Its funny how people keep saying Sanji being kind is special attribute which only applies to him, but forget that he was ready to leave the kids in punk hazard behind had nami not convinced them. Even Zoro let himself be captured for the sake of that little girl, Chopper is one of the kindest sh out there. Luffy saw Zoro and Sanji's actions before asking them to join his crew, he is kind in his own non pretentious way albeit mostly to the people he likes.