So here's what i think happened:
Roger told Shanks about Joyboy and the Gomu Gomu no Mi, and he asked Shanks to find the fruit and give it to his son, and protect him
Shanks was conflicted, because he also wanted to become the Pirate King, but his hero Roger is telling him that it needs to be this fated Joyboy, and it isn't him, but Roger's son
Shanks ends up accepting it
Shanks finds Ace, and that's why he spent 1 year in Luffy's island, keeping an eye on Ace from afar
he ended up meeting Luffy and Makino there, while just checking on Ace
He finds the Gomu Gomu no Mi and collects it
I think Shanks was gonna ask Makino to keep the fruit with her and give it to Ace when the time was right, or give it to his own son depending on what happened
but Luffy ends up eating it by mistake
Shanks decides to bet on Luffy, because he sees how Luffy is similar to Roger and had the same words, instead of Ace who hated his dad.
So he saves Luffy, instead of just letting him die and get the Devil Fruit when it was reborn.
Shanks decides to wait for Luffy to awaken as Joyboy.
Now, finally, his promise with his captain Roger is done and he can try to become Pirate King like he wanted.
so Shanks is gonna try to go against fate.
That's why Shank's Conqueror's will is so strong.
He wants to be Pirate King and be at the top, but he knows about the destined Joyboy, yet he hasn't given up. Like Kaido did, Kaido gave up once he find out he wasn't Joyboy.