User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-4933386-20170205092919/@comment-5303512-20170205132458

It could have been the result of when Zeref pierced the book that the seed developed.

But that emotion thing does not make sense, Natsu just learned about it a while ago and he seemed to have gotten over it when he Happy decided to still be friends.