User blog comment:BlackWizardZeref/Fairy Tail Chapter 394: Juvia vs Keith Review/@comment-99.14.133.213-20140801053136/@comment-9595671-20140801113233

In my opinion it's because Zeref said that Natsu being E.N.D.'s vessel doesn't sound believable. Even if they inhabit the same body, Natsu and E.N.D. would both be meeting Zeref because while the inside is E.N.D., the outside is still Natsu.