User blog comment:Renkrisb/The Dragneel Brothers/@comment-25618663-20150519141305

I do wonder about how Zeref, Natsu and Igneel are (possibly) all connected! I think Zeref let Natsu cause havoc upon the dragons to test Natsu's strength, let's say using his Etherias form. I think a fight between Igneel and Zeref happened. Igneel couldn't win, and asked for a truce. Zeref could feel how powerful Igneel is and asked Igneel to teach Natsu dragonslayer magic, etc.

My theory anyway. xD