Is there some type of connection between Mavis and Zeref?
I think Zeref and Mavis had a thing. You know how Zeref said one day, that he realized the meaning of life and stopped all of his bad ways? Well, I think that day was the day Mavis died. Possibly killed by Zeref himself. He may have felt terrible and realized that he needed to be stopped. So as he awaits Natsu to get stronger, he stays near Mavis, enjoying her presences till the day comes.