User blog comment:Water Dragon Slayer Shawn/Theory on why Gajeel and Natsu couldn't pass Freed's barrier./@comment-173.2.196.229-20141228051939

(Spoilers)

How do you think Zeref remembers Natsu? Natsu clearly has no memory of Zeref at all, but the one thing that could be true is that Natsu and Gajeel were both around Zeref's age, (400+) and both of them had been sealed away for four hundred years with a spell similar to Fairy's Fear (The spell casted to protect the FT members on Tenrou from Acnologia in the Tenrou Island Arc, though sealing them away for seven years when a week had passed for themselves), and they awoke with no memories besides being taught by Dragons.

The reason I say specifically Natsu and Gajeel isn't because they were the only two Dragon Slayers in Fairy Tail at the time (Excluding Laxus as he's Second Generation), but because Zeref remembers Natsu, and Natsu and Gajeel's ages aren't given on their wikia pages on this website, as the rest of the Dragon Slayers (Second and Third Gen. included) have their ages shown, yet it's never said in the Anime or Manga how old they are. Just the wiki.

This leads to the theory of E.N.D. being Evil Natsu Dragneel. In Chapter 413, Igneel said the Dragons were sealed away into their foster children's bodies for two reasons: To prevent them from turning into Dragons, such as Acnologia did, leaving the second reason in the dark as Igneel was interrupted by a rising Acnologia after he was knocked to the ground, Igneel landing on him. I personally believe Igneel's second reason for entering Natsu's body was to prevent him from becoming Evil Natsu Dragneel, a.k.a. E.N.D. The other Dragons must have entered their slayer's bodies for only one reason, leaving Igneel with two reasons.