User blog comment:IamJakuhoRaikoben/Chapter 451: Fairy Heart, Review./@comment-4449950-20150907045807/@comment-7716957-20150907051719

I think Zeref views Acnologia as his ultimate rival in terms of destruction. Like "let's see who's more powerful to kill more?" kinda competition.

Meh idk.

Perhaps some bad blood between those two to want to have some ultimate battle.