User blog comment:IamJakuhoRaikoben/Chapter 450: Alone in all the World, Review./@comment-24800928-20150831123618/@comment-9595671-20150831160648

I'll buy that idea, considering that their minds were likely both a bit unstable over the whole thing the moment seemed almost like a fight or flight reaction.

Mavis stated she'd understand him, they were both immortal, they would be the only ones each other had in a world where they'd take life and never die... that's probably another problem on Zeref's end.

He thought he couldn't lose Mavis, but then a miracle of a contradiction ends her life while he is still living. He essentially had a hope spot, gained hope only for it to fade and bring him despair.