User blog comment:Dtar/Dragon Force Theory/@comment-180.191.130.39-20170109221802/@comment-31021015-20170111192445

Don't forget that may just have been because Natsu technically using not just dragon slaying magic but his demon slayers as well. (for the dragonifacation to turn people they have to use the slaying magic enough to actually make it happen. So the entire time Natsu uses fire he must be subconsciously using his demon slayer magic considering the demon seed is already taking effect on him.) Which explains his unfailing strength throughout the entire series.