User blog comment:FairyRaven/Fairy Tail Chapter 448: Fight the Power! Review and Commentary/@comment-24951162-20150817141923/@comment-4829046-20150817143656

If you mean besides the usual fun-shipping condition of 'Who Cares why lol' then it's cause when introduced, people wanted to know their history and it wasn't far-fetched to assume they were lovers because at the time, we weren't even sure how old Mavis was or why Zeref chose to be on the Fairy Tail holy island. Then we saw them actually meet and Zeref seemed to be connected to her but nothing romantic was seen so some shed the ship. After Fairy Tail Zero, the opinion differed even more because we saw their first meeting and whatnot and that they were master and student, and alos that Mavis(though jokingly) proclaimed herself a Dark Mage.

I have a feeling that the ship will sink or sail once we see what Mavis has to say.