Dark themes. Read at own risk!
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When it comes to Sasuke Uchiha, there is a whole side of selfishness involved in despising yourself.
Now, masochism itself (suicide, specifically) is not describable as inconsiderate: One might think it terrible to leave kids, spouses, or loved ones behind for a person's own relief. However, the verity of the matter is that these loved ones are the people who keep the depressed around for one more day. In many cases, the suicidal think of their loved ones until the very last second. This soul crushing, spiraling depression consumes them to the point of no return.
In most cases.
(Things are always unusual when it comes to Sasuke Uchiha.)
Naruto saw what he did: The manufacture of a flawless smirk, a graceful stride and calm, calculating eyes that looked straight ahead without concern.. His almost flawless imitation of laughter, stifled by a hand and dismissed by a 'hn'. That laugh was foreign.
Naruto ALWAYS saw through Sasuke.
It had started as suspicion; Naruto's simple inclination of the ear when he had shoved his best friend slightly too hard, originally believing he was fine until hearing the small 'oof', without feeling a backlash shove or at least hearing a sharp retort.
It made him think about Sasuke a bit; the blows he used to make and the intensity of their childish squabbles before Sasuke had abandoned everything he was at age thirteen. The infinitely different Sasuke that he knew now. And, that ever since his probation, how Sasuke never once threw a punch.
So Naruto came to realize, that yes, Sasuke was a masochist: Not in the traditional sense or in the sense that he realized what he'd done and felt guilt for it, choosing to be kind from now on: No. Sasuke was a living mirage, an unlineal product of his old self, hiding the deepest cracks in his scars with cheap tar that only left a deeper wound.
Naruto watched his best friend suffer. On Friday nights Sasuke would sit in dangerous spots at the bar, provoking anger from drunken shinobi in the event that people had been decent to him that day. And Naruto had to watch as these men threw hits at his desperate friend, often times becoming so infuriated that he himself would step in and beat the sh*t out of them, only so that he could yell at Sasuke for being so careless.
"Sorry. I'll watch out next time."
He never did.
Perhaps the real punishment he was seeking was from Naruto. His cries on Friday nights of "DON'T BE SUCH AN IDIOT! I'M TIRED OF CONSTANTLY HAVING TO BAIL YOU OUT OF THESE THINGS, OKAY?!"
That was the first time Naruto landed him one. Not out of frustration or true anger- no. Out of sympathy. He felt so desperate to help Sasuke, that he'd hurt him if it was enough to relieve his spiraling guilt.
(Was it ever enough?)
It was Sasuke's therapy; the only way to escape his pain was to throw it at someone else; to finally, for ONCE be able to pity himself. But these sessions had escalated quickly. In fact, often times Naruto felt his knuckles go numb from throwing unreciprocated punches, Sasueke's desperate looking eyes never once faltering to allow Naruto to hold back. It hurt Sasuke. It hurt a lot.
Hurting Sasuke, though, hurt Naruto. It left him with bloody, bruised knuckles and tear stained cheeks, red from heat, and exhaustion, and sadness.
So, yes; Sasuke was a masochist.
But Naruto was the martyr.