User blog comment:FairyRaven/Fairy Tail Chapter 325: Unity, Review and Commentary/@comment-49.176.6.51-20130329100223/@comment-7418318-20130329100917

It was explained by the soul of the dead dragon that Wendy called. There was a big civil war between the Dragons that thought of Humans as either food or vermin and those Dragons that wanted to peacefully co-exist with us. Since the war was a deadlock the pro-Human dragons taught the Humans dragon slayer magic.