User blog comment:B214/Question and Answers 6/@comment-24098614-20170326152005/@comment-190.160.185.214-20170327000811

it's  an important factor the reason why they're getting stronger, Issei is getting stronger because he must, because he's a hero and a he needs to protect a lot of people, Vali does also want to protect people, but he mostly gets stronger in order to gain supremacy (hell, the moment he got his resolve to be strong to protect, instead of being strong just to be strong was when he unlocked his DxD L form)....that's why Issei got strong so fast, the motives for which one grows strong normally dictate how fast one gets strong