KingSanchezD

In One Piece, Roronoa Zoro promised to be the world's greatest swordsman to his childhood friend, Shimotsuki Kuina. 

Explanation 

Zoro and Kuina would often spar, but Zoro never won.They vowed that one of them would become the world's greatest swordsman.Tragedy intervened before their promise could be fulfilled.

Zoro has also revealed his dream of becoming the world's best swordsman to Luffy. Luffy supported Zoro's dream, believing that the Pirate King should have the best in his crew.  I think that is all we need to know.. He's a man of his word.. He had his heart broken young, and doesn't need layers he made a promise young, and that's all that needs to be said as draws on that. He was ready to die before he met luffy seeing him grow is enough for me... I'd be. Disappointed if oda sold out and changed anything.. Not every thing needs to have layers or some legendary back story. All captains have their dogs. Not dog in a negative sense byt a dog as in follower, best friend. Dogs are the loyalty and backbone and strength that a captain pushes forward for. A captain without his dogs or his crew is an example of why someone like Dracule Mihawk exists.