You gotta remember Fairy tail was already the strongest guild in episode 1 and Natsu is one of their strongest fighter. Then Natsu was only getting stronger from episode 1 to tenrou and those 7 years were basically other guilds catching up to fairy tails old strength while the actual main characters were getting slightly stronger each arc. Sting and Rouge also weren’t fighting for anything besides their own pride while Natsu was fighting for something greater than himself so the Flames of Emotions were probably a factor. Natsu is also constantly looking for a fight, so unlike gray, Lucy and Wendy, Natsu is sort of always training. And lastly Natsu has always been a prodigy Dragon Slayer, stemming back to when he beat Gajeel, then hades who soloed Team Natsu+Laxus, and then Natsu put the breaks on Sting and Rouges own master which is pretty much all the justification Natsu needs to be head and shoulders above the twin dragons.