User blog comment:LastationLover5000/Fairy Tail 283 - In Which Nakama Is Spoken (Natsu vs. Sabertooth)/@comment-98.224.85.130-20140608100755

It may seem like an ass pull that Natsu and Gajeel can eat other elements besides their own and power up. But it fits with the Dragon Slayer magic. A Dragon Slayer can eat their own element and get stronger, but if you go by that logic the Dragon Slayer magic is really only efficient against the Dragons that taught the Dragon Slayers. Which means that the humans would be killing their parents/masters. But it is possible that is only for low level Dragon Slayers. And the more advanced one gets at their magic, the Dragon Slayers can eat other elements. Which makes more sense, if a Dragon Slayer was to fight a random Dragon, and could only eat their only element, their magic is only efficient at regaining their strength. But if the Slayers could eat other elements, they could eat the Dragons attacks and only get stronger, giving them a huge advantage. It is possible as although we have no idea what Natsu and Gajeel's ages are, they looked like children when their respective Dragons left. So it is possible that their Dragons only taught them the basics, and only know are Natsu and Gajeel reaching the more advanced levels.