A strong user of Conquer’s Will can manifest his spirit in such a way to make people feel, think or act as he desires. This could be only partly effective on more individualistic and strong person and is not out right mind control.
This would explain why Zoro first turned down the invitation to join, why both Nami and Robin couldn’t leave the crew, why people close to Luffy proclaim that he will be the Pirate King and cities of people try to protect him.
But not only Luffy can do this. Blackbeard too made instant allies out of Impel Down prisoners. Whitebeard had all the surrounding people acknowledge him as their father.
And Roger himself by just telling about a live of adventure to a lazy stranger convince him to journey the world round and be free as can be, even though Rayleigh first laughed at him as a fool.
A true King shapes the minds of the ones following him.
*Theory by Kizoku. LyLord
