User blog comment:FairyRaven/Fairy Tail Chapter 414: Drops of Fire, Review and Commentary/@comment-7418318-20150105145742

I just had a scary thought...

Natsu might think that if Gray had relinguished the book of END, he would have been there to fight alongside Igneel and together they'd have beaten Acnologia. I know that it's highly doubtful that the two actually could have beaten Acnologia, but in his grief Natsu might end with that conclusion.

If that's the case it could easily come to blows between them, especially if Gray answers with an insult to Natsu's accusation.