User blog comment:IamJakuhoRaikoben/Chapter 497: The Winter Mage, Review./@comment-24951162-20160810202218/@comment-5303512-20160816042228

Perhaps like you said because of Natsu being unique to the other Etherious that he would have to develop slowly at an early age so he wouldn't have the power right away. We cannot say for certain that in that Book it is what you say it is, it could be that Igneel had to remove Natsu's Etherious powers into the form of the book and rewrite his memories about it and that Natsu opening and reading it would release that power like Carla said, each book contains the means of summoning demons. And this part is just a thought but Natsu as E.N.D. Hated Zeref and didn't want to kill him just to make him suffer by living forever, because Natsu has a right to live. But again, there are things that are unaccounted for and that we can't say for certain, all we know really that Mard Geer lied about E.N.D. Creating Tartaros. And as far as Curses go, that would go back to my point about Natsu/E.N.D. hating Zeref for what he made him to be and that could be where it started for curses.