He didn't wear the mask due to feelings of superiority or even inferiority.
He wore it because when he was four years-old, on the anniversary of his birth (and the anniversary of his mother's death) his father had accidentally looked at him for a split-second too long. Though he was a fine shinobi (and a decent father), Sakumo had not been able to disguise the spark of terrified realisation.
That Christmas, he'd received more scarves than toys or weapons and his father told him to bundle up with paternal warmth. That warmth had not been quite hot enough to burn away the pain that lingered in Sakumo's eyes.
So much like your father, people cooed at him, all through his life, in mockery or in awe or in hatred, even now.
So much like your mother had been written all over his father's face, even as his lifeblood had dyed the soles of Kakashi's sandals burgundy.
He still wore it because despite being a supposed prodigy, some kind of genius, a crack ran through the surface of his face, an angry welt which threatened to crumble into a canyon.
If one pressed close enough to the surface and looked, then within that crack would be his childhood, when he had just been a boy and not a boy on the battlefield. Laughter echoed within the gorge, as pure as the sound of a glass bell but with no edge or sharpness, distorted eerily as it bounced off jagged walls.
The laughter of the dead.
So he continued to wear the mask, schooling himself to convey a variety of expressions using only the upper part of his face. People seemed to understand him just fine. You bore me. I don't want your admiration. Leave me alone. I don't want your pity.
I don't need your pity.
Every week, he spent at least two hours meditating, pouring cement to reinforce the edges of the narrow ravine, willing the cracks to splinter no further.
He'd left ANBU because the extra mask had made him careless. One day, he'd found himself smiling as he disemboweled yet another shinobi. The scent of fear, rank and putrid, had awakened something ugly within him, and this creature had clawed its way onto his face.
He wasn't, as most people assumed, emotionally and socially backwards.
He can be impeccably polite if he wishes, perfectly amiable and completely personable. Considerate, even. He can read the desires of others through their eyes and hands and bodies, his being practically built for diplomacy.
He simply chooses not to. He wraps compassion and mercy under layers of duty. He moulds his feelings and regrets into regular, rectangular shapes and slots them into their places in the gorge that divides the landscape of his mind. The regrets are pushed in deeper because dwelling on them is a luxury, an indulgence he cannot allow himself.
Every act of kindness performed is a further temptation to be kinder to himself.
Gai may claim to be a beast, but Kakashi knows that he is the true monster.
It's a good thing he's so skilled at pushing unwanted things off the sheer cliffs. Every time he shoves the monster into the seemingly bottomless chasm, it hibernates for a time, nursing fractures and licking at wounds with its slavering tongue-
-until it climbs back up.
"Kakashi-sensei!"
He turned at the familiar voice, removing a hand from his pocket to wave at his former student. Automatically, his eyes curved into their usual smile even as his lips remained rigid, unchafed by the rough fabric of the mask.