Never let them see your pain. That's one of the first rules Naruto learned when he was younger. Because if they see it, they'll take advantage of it, until you're nothing but a blubbering, crying ball of misery. And they'll laugh all the while.
Somehow, their laughter makes it so much worse.
So you have to hide it, hide your pain, and the crying, screaming child inside of you. You have to put on a mask and perfect it so they'll never see through it. What kind of mask, you ask? There are many. Naruto grew up in a hidden village full of ninja. Almost everyone had some kind of mask. There was the stoic mask, the angry one, the indifferent one, the superior one, and many others. Naruto's favorite was the smiling mask. It looked so innocent and innocuous. It was one of the least used, Naruto had only seen it on one person, a traveling ninja, and he couldn't figure out why. It was perfect.
The next day, Naruto started working on his smile. It was hard at first, his mask kept breaking, but it got easier quickly. And it worked! A part of Naruto was surprised, hadn't thought that it would actually help him. Of course, the abuse didn't stop, but they tired of it easier now, and often it stayed just in verbal, and didn't go into the physical. But some of those comments hurt more than a punch to the face would. Naruto almost wished they would just beat him up instead of yelling, but none of his feelings got through the mask, so his tormentors knew nothing.
The mask stopped their laughter, their horrible, mocking, taunting laughs that had hurt most of all. That was what Naruto was most thankful for.
It took a while, surprisingly, for anyone to notice his mask. You'd think, in s hidden village, that people would notice masks, even if it was just a mental note. But no one cared to look at the demon child when they weren't torturing him, and so no one noticed it.
The first person to notice it was Kakashi. Naruto had been expecting it though, because Kakashi not only wore a perverted mask, but an actual physical cloth one as well, so he obviously knew what masks looked like. He was the first person to look deep enough to see it. But that was ok, because Kakashi understood the necessity of masks, so Naruto knew he wouldn't tell anyone. His mask was still safe, and he smiled all the while.
The second person to notice was Sakura, but it was only for a moment, fleeting, and she forgot soon after. His mask had broken, just for a second, in front of her, leaving him open, vulnerable, wounded, his eyes full of so much pain and emptiness it was enough to make anyone's heart break. She had stared, had started to walk towards him, but something had distracted her. She had looked away, and a few minutes later, the incident had been relegated to the back of her mind, forgotten. So his mask was still safe, because you couldn't tell what you couldn't see or remember.
Sasuke noticed because he too wore a mask, although he had chosen an indifferent and superior one. That combination had pissed Naruto off, so kept trying to break it, and almost came close a couple times. Sasuke was the first person to break his mask, really break it, so that it was almost hard to put it back on. But he did it, because if they knew it hurt, they would take advantage of that weakness and make it hurt even more.
Iruka is the only one who can make his mask different, because it's only been with Iruka that his smile has ever turned real. It felt weird, awkward almost, the first time it happened. He had touched his face to make sure he was still smiling, and he was, but it was different. That smile had felt good. After Iruka had seen it, he had gone quiet for a moment. Every time after that, when he saw Naruto's mask smile, he did everything he could to make it real. With Iruka, the screaming child inside that was crying 'help me help me, why can't you see' was quieted sometimes, with gentle words and comforting hands. It is because of Iruka that Naruto even knows the difference between a mask smile and a real smile.
Because although it was hard at first, constantly keeping the smile on his face got easier. He found himself waking up with the smile; found that sometimes he even forgot it was there, the mask had become such a part of him. He had figured out, though, why ninjas rarely used the smile as a mask.
It hurt sometimes, to be always smiling, and never happy, and never have anyone care enough to see through it and try to help.