Ok, here's a theory. What if, and this is a big what if, but what if Luffy isn't Joyboy, but the fruit itself is. We know that devil fruits are said to contain the souls of devils, and those souls had to have come somewhere. A soul is a tangible thing in the One Piece universe, a fact proven by Big Mom's fruit. And we know that based on the Straw Hats number Theory, that luffys fruit can also be called the Nika Nika no Mi, similar to the Sun God Nika, who's said to BE Joyboy. We also know that luffys fruit is VERY important to the world government. What if the ancient kingdom created the Devil Fruits, they figured out a way to infuse their souls Into fruits, to preserve themselves as they were going to lose the war. This also ties into the Will of D. What if the will of D represents the people who can fully utilize the soul in their devil fruit. Luffy lost his 'voice', and that, combined with the massive amounts of damage he's taken, has allowed him to fully 'awaken' his fruits soul.

TL;DR: Luffy isn't Joyboy, his fruit itself is Joyboy