User blog comment:Sargenttacos115/Why are Sting and Rouge so weak?/@comment-7418318-20131109165310

Maybe they are just slackers when it comes to difficult missions and training.

We don't know much about Gajeel's choice of missions, but for Natsu they couldn't contain enough fighting against strong opponents. Along with his constant brawling with guild-mates and fighting against enemies like Gajeel, Cobra, Zero and Hades, he certainly got plenty of opportunities to improve his fighting abilities, even without putting Second Origin into the mix.

In addition, Dragon Slayers gain power from their feelings. Prior to the tag battle in the GMG, the twin Dragons for some reason didn't know that and thus were unable to use it to their advantage. "The Power of Feelings" is what Minerva called it... "Nakama crap" is what many readers call it :P

Either way, with Natsu never supressing his emotions from the very biginning, that gave him another edge over the twin Dragons.

Gajeel also seems to live out his emotions to me. Even though he hides them behind words, his deeds seem very unrestrained.