User blog comment:Sargenttacos115/Natsu and Gajeel age Theory/@comment-25324252-20150213191521/@comment-71.50.218.122-20150222075641

Actually dragons being in them was not the only reason Natsu couldn't pass through. Natsu is actually one of Zeref's demons, he's actually E.N.D. It stands for Etherious Natsu Dragneel. dragons do explain Gajeel however