User blog comment:Miskos3/Fairy Tail Chapter 505: Trump Card, Review/@comment-97.104.204.83-20161010124046/@comment-84.197.227.90-20161011081111

I think Mavis contracted the Curse due to using Law.

It is likely a Magic developped by Zeref and it acts quite similar to the Curse. Law targets those the caster deems enemies, the Curse targets those the cursed deems friends/loved ones. Zeref may have created it in orde to apply the Curse's effect on those he considers enemies, as the real Curse wouldn't kick in.

It might even be a spell distilled from the very machinations of the Curse itself, therefore including the risk of the caster contracting the Curse. Perhaps Anhkseram wasn't too fond of Law's creation and decided to curse whoever casts Zeref's Magic.

Mavis later modified it into Fairy Law and had it consume the caster's life in order to avoid the caster getting Cursed by Ankhseram.

She probably didn't want anyone else to contract the Curse after what happened to Yuri's wife. Fairy Law achieves this, since casting Fairy Law is actually only making light of one's own life and Ankhseram shouldn't have any issues with someone willing to die sooner.