User blog comment:IamJakuhoRaikoben/Chapter 373: Life or Death?, Review./@comment-24536822-20140228172918/@comment-7418318-20140228180617

We can only speculate about that.

Maybe there's a prophecy that foretells that only Natsu and E.N.D. can reach him, or maybe Zeref's hanging out in a palce that can only be reached by going through a volcano, which would make Natsu the only mortal with a realistic chance to get there.