Ok, I already posted a picture at least proving that Natsu knew what love/like/crushes were, but I am not joking when I say I have re-read the entire series, from whats out, about 5-6 times over. And I have every chapter downloaded on my phone, so I can re-read it again when I'm bored with no wifi.
Now, I'll try, as always, not to let my fangirliness get in the way here, but I have been finding more and more moments, just proving how he's REALLY not as dumb as everyone thinks.
I have seen moments where he was able to piece things together, like on Galuna Island, when he tilted the ruins, and figured out Iced Shell was the magic the would "kill" you, without anyone telling him. Now I GET he's not the brightest bulb on the Christmas tree, but he's not completely lost either.
After a LOT of observing, he mainly just seems more naive to the world, and doesn't have the best memory. Because he WAS raised by a dragon, and he was mainly taught fighting, probably to beat END and Zeref, and Acnologia, AND ever heard people say; "it's the teachers fault, not the student". I had a friend say "Natsu probably wasn't the easiest to teach" ok, yeah, but maybe Igneel wasn't the best at teaching him everything about humans? and maybe he focused on more necessities since he DID have to leave Natsu rather shortly.
Also, memory doesn't mean smarts. I know that I don't have the best memory when it comes to a LOT of important things in school. BUT I can practically recite Fairy Tail from front to cover. But I am by NO MEANS an idiot. I'm still a logical thinker, and I have impressed my people with how well I can think things through. I'm not perfect by any means, but I'm still pretty damn smart. And this could also be the case for Natsu.
Also, it's become PRETTY OBVIOUS he's a genius fighter, and DOES use strategy lots of times to win fights. And I have UNDENIABLE proof from one of the more recent chapters, where Natsu used a giant boulder to crush Franmalth, because inanimate objects have no souls, and he couldn't stop it.
AND THERE'S MY PIECE.
I hope you really consider everything, and really take all these points into consideration. I'm not saying he's a freakin' genius, but he's not completely lost to logic.