Did Natsu Really Say “The Moment You Think of Giving Up, Think of the Reason Why You Held On So Long”?
No. The sentence was circulating as an anonymous English motivational line by 2012, attributed to nobody; it was pinned to Natsu Dragneel, and then to Hiro Mashima, years later. Natsu's real never-give-up lines are in the manga and are blunter.
No. Natsu Dragneel does not say "the moment you think of giving up, think of the reason why you held on so long" anywhere in Fairy Tail, and Hiro Mashima did not write it. It is an anonymous English motivational sentence that picked up an anime character the way a magnet picks up filings.
The verdict
Misattributed. The line is real in the sense that people have been saying it for years; it is not real as a line of Fairy Tail. Wikiquote's Fairy Tail page, which transcribes Natsu chapter by chapter, has no trace of it. The quote databases that attribute it to him give no chapter or episode, and the ones that attribute it to Mashima give no interview or volume note. Nobody has ever pointed at the page where it appears, because there is no such page.
Where it actually came from
The sentence predates its anime attribution. A Chinese Q&A site has a student in March 2012 asking for help writing an English speech around "the minute you think of giving up, think of the reason why you held on so long," with no author named, and Tiny Buddha's quote archive lists the same wording under Unknown. The "minute" version is the older one; "moment" is the variant that later got attached to Natsu. A near-twin, "if you ever think about giving up, remember why you held on for so long," circulates under the name of Paramore's Hayley Williams on the same user-submitted databases, again without a source. It is a floating aphorism with at least three claimed owners and no verifiable one, and Natsu is simply the owner the anime side of the internet settled on.
Why it stuck
Because Natsu is the shonen genre's loudest refuser of defeat, and a quote about not giving up looks like it belongs in his mouth. The anonymous version is smooth and reflective; the actual Natsu is neither. When quote pages needed a Fairy Tail entry with the polish of a greetings card, the floating sentence was available and the character's reputation did the rest. Mashima's name got attached later still, presumably by someone reasoning that if Natsu said it, the author wrote it, which would be sound logic if the first half were true.
What he actually says
The real lines are on the record and are shouted rather than reflected upon. In chapter 194, beaten down and getting up again, he says "I will never give up!" and repeats it until the panel runs out of room; in chapter 276, during the Grand Magic Games, he ties his own refusal to the guild's, insisting that Fairy Tail never gave up and so there is no way he will now. Those are the sentences a reader of the manga would recognise. They are not quotable in the poster sense, which is exactly why the poster sense went looking elsewhere.
How we checked
Our verification process starts by finding the line in a primary or editorial source, and for this one the search ends at an anonymous 2012 speech prompt. The Fairy Tail material on Wikiquote is chapter-numbered and does not contain it; the attributions to Natsu and to Mashima are all on user-submitted databases, none with a citation. It joins the fact-check database as the clearest case we have of a quote that existed first and found its character second.