I never said they had to be manifested from struggles. I just gave a possible scenario of a strong desire, through a struggle.
We will go back to Zoro. He desires to be the strongest Swordsman. Asura is an ability that clearly takes him to another level of strength as a swordsman, by allowing him to utilize a Nine Sword style of fighting. If that is indeed the route that Oda is going with Asura, then clearly it's not something that just manifests in the blink of an eye. It takes time.