User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-182.68.37.230-20151209114329/@comment-7418318-20151209153139

Zeref did start his obsession with reviving Natsu before he became immortal... actually it was this very obsession that led to him being cursed in the first place.

Maybe he didn't yet know he would live an eternal life back when he turned Natsu into E.N.D.