User blog comment:FairyRaven/Fairy Tail Chapter 448: Fight the Power! Review and Commentary/@comment-5840892-20150817232654/@comment-7418318-20150817235008

Several reasons:
 * 1) Despite her being 23 when she died, she had the body of a 12 year old. I seriously doubt Mashima wants to go there... and even if he did, I doubt the editor would let him.
 * 2) Zeref had this whole death-curse going on, remember? Staying too close to him was inviting death, so any romance would have had to stay at a distance. Having to keep several meters away from your "lover" isn't going to be very good for the relationship.
 * 3) The contradictory curse itself: The more Zeref values life, the more out of control his powers go. And loving a living being is pretty much the zenith of caring for life. So truly loving Mavis would have made it all that more unlikely for them to spend time together.
 * 4) There's a more than 350 years gap between their ages.