User blog comment:IamJakuhoRaikoben/Chapter 449: Mavis & Zeref, Review./@comment-7418318-20150825065620

Picking up on a theory posted several times below:

Some people seem to believe that Mavis' curse is somewhat opposite of Zeref's and giving life instead of taking it. While I've seen nothing that would convince me of it, it would make a nice story device.

If the two curses are really opposites, maybe they'll eventually cancel each other out.

As for why: It's been 400 years of constant torment for Zeref. Maybe Ankhseram is finally starting to think that Zeref was punished enough, so he provided him with an opportunity to break the curse.

But as we've already learned from the very existance of the curse and the reason behind it's being brought down on Zeref, Ankhseram is quite an asshole (if he's really the source of the curse to begin with, but that's a different topic), so he couldn't just lift the curse, but only provided a means for Zeref's release but left it to him to figure it out.

It'd be both a release and another form of torture though (which would fit right in with the theme so far). Either don't love your equal and opposite and thus continue to eternally kill people. Or allow yourself to love the equal-opposite, which results in both him and her dying.