User blog comment:FleetAdmiral88/The Most Worthy End Fight/@comment-5015945-20120629015847/@comment-5015945-20120629193200

Because Shanks made Luffy want to become a pirate. He inspired Luffy to begin his journey to become the pirate king and he has to go through Shanks to become the next king.

Luffy to Shanks:I'll gather my own crew and beat yours and find the world's greatest treasure! No matter what I will become the pirate king!