Why straw hat was given to Shanks ? - One Piece

              

             Straw Hat is Luffy's most prized possession that Luffy wears every time, it was given to him by his friend and source of his inspiration to become a pirate "Red Haired Shanks" also known as "Akagami no Shanks",later in the story it was revealed that the Straw Hat originally belonged to none other than Pirate King "Gol D Roger", so the question arises that why would Gol D Roger gave a straw hat to his subordinate as a gift unless there is a reason or promise associated with it.
                                          

   What was the promise , Well I think the promise behind passing that Straw Hat to Shanks was to return that hat back to Gol D Roger after achieving something huge , While Gol D Roger was alive , Shanks was never able to return that straw hat to Gol D Roger , He was shown holding that straw hat while crying in execution of pirate king. Certain years after Roger's execution, Shanks became a pirate known throughout the world when he claimed status of a "yonkou" or a "Sea Emperor", after achieving that feat he decided to return that Straw Hat to Gol D Roger's grave in east blue, although whereabouts of pirate king's grave are unknown but it can be considered that he was not buried in Loguetown because otherwise we would have known it in Loguetown chapters/episodes ), he wanted to go to his grave but he planned to
do an important work in Fuusha village first (it is unknown what work it was or why he stayed in that village for 1 year), but there he met Luffy. As we have seen Luffy has same appearance as well as personality as Gol D Roger. While Shanks spent time with Luffy, Luffy always used to reach out for that Straw Hat, also Shanks saw some kind of spark in Luffy that remineded him of Pirate King. 

     In Saboady Arc, it was shown in Reyleigh's flashback that Luffy said exact same words as Gol D Roger . It is still unknown what those words are but those certainly are not "I am gonna be King of the pirates" because in great pirate era everyone screams those words. But certainly those words compelled Shanks to pass down that straw hat to Luffy as if it was his destiny to wear that Straw hat and Shanks made the same promise with Luffy.

              Above written article is a theory and It's just What I think might've happened. If you have any other suggestion please tell us in comments.

Thank You for reading.