From an interview with Mashima-sensei in Weekly Shonen Magazine 2010 Issue 15, where he shares 4 tips on creating characters.
Why Natsu never has any monologues in the manga
Natsu’s an extremely energetic character, and you never know what he’s up to. I wanted Natsu’s actions to make the readers think “this guy is troublesome”, or “he did something interesting”, and let them laugh and be surprised. With that in mind, I do not include any monologue from Natsu in the manga so that the readers will not know how Natsu will act.Article ©Mashima Hiro · Kodansha
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Mashima-sensei has stated more recently in a tweet that even he himself doesn’t quite understand what Natsu might be thinking.