User blog comment:FairyRaven/Fairy Tail Chapter 448: Fight the Power! Review and Commentary/@comment-24546844-20150817211613/@comment-24546844-20150818002936

I don't think Zeref's and Mavis' relationtionship is romantic or if it was I doubt it was mutual (it would be interesting if it was though). My guess is that Mavis started searching for Zeref after creating FT to thank him, then he told her about his story and together they started searching for the one magic is hopes that it would free Zeref from his curses and while researching Mavis died acidentally after finding the One Magic's true nature and not being able to handle it. That explains Zeref's guilt ridden narration of FT Zero.