Shanks is the most mysterious Emperor in One Piece.
He can appear at different places in a really fast manner, he met with Gorôsei almost in a non-chalant fashion and he even protected another Yonko. But why? I think I have found the mystery behind, if anything, Shanks’ motivations.
First of all, I think Shanks grew up in a war-torn country. I came to this conclusion since Shanks is a man who wants to avoid conflict at all costs. In Luffy’s hometown Foosha, he only acted when someone hurt a friend of his. He stopped Kaido to prevent the Summit War from getting out of hand so that the world wouldn’t be thrown into further chaos. Even before that, Shanks might have foreseen that Blackbeard—someone who gave him a permanent scar—would beat Ace if the two were to confront each other and this would lead into a disastrous war.
In his portrayal of Shanks, Oda shows him mostly drinking and partying, painting a picture of a laid-back man. This, in my opinion, ties into his past as an orphan from a warring state. This is pure conjecture, but bear with me.
Shanks knows the truth about the world of One Piece. He sailed with Roger for a time and he is one of the Yonko. Now, we need to remember the words of a dying Whitebeard: “. . .and in the future, someday, someone will appear, carrying the history of all those decades on his back, and challenge this whole world to a fight. Sengoku, you guys in the World Government, all fear that immense battle engulfing the whole world that will eventually come. I don’t care myself. . .but as soon as someone finds that great treasure the entire world will be turned upside down. And someone will find it. That day will come sooner or later.”
We know now, from his dying words, that Whitebeard knew the secret behind the world of One Piece (and the flashback). This lets us say that, even if Shanks doesn’t know this “secret”, he is now sure that finding One Piece will lead to war, as Whitebeard was the name of the era, his words must be trust-worthy. And Shanks is afraid of this war. He wants to avoid it at all costs.
And what would a man who wants to avoid this war want? One Piece to not be found. He rejects the Pirate King. What happened to his mentor, Roger, when he found the One Piece? He died. He lost his mentor, maybe his father figure because of his pursuit of One Piece. So, Shanks doesn’t want there to be another Pirate King.
One thing you could say is that Roger’s death had nothing to do with finding One Piece. However, Shanks might not see it that way, or even if he did, it wouldn’t be impossible to think that, in a war caused by the excavation One Piece, many of his friends, his protegé would die. He would be sucked into the war-torn hell he lived in when he was young.