It's an old tradition Iruka teaches him in the bar, the morning after. Too early for drinks, but in a life of too lates, it works out.
A teacher's tradition. One shot for each student lost.
Five other teachers join then, Asuma watches, no drinks and only the stale taste of cigarettes to remind him of the bitterness of life. One old teacher doesn't seem to have enough, and Kakashi is told:
When you can't drink them all, that means it's time for you to let go.
This can't be healthy, but Kakashi watches as Iruka lines up the bitter, bitter shots, more poison than pleasure. Iruka names all five. He rotates through names on different days, because Iruka hasn't learned how to let go. Each name has a face. A story. A death.
Kakashi doesn't want to know about them.
Instead, he taps the cold glass on the counter. Everyone stares at him, and he drags the mask from his face.
Sasuke.
One motion, bitter burning bit of a taste, blazing an acid trail all the way down as he swallows. Kakashi's eye waters, and Iruka something between slaps and pats him on the back. Camaraderie in failure. In grief.
Kakashi feels no better, no closer to absolved. He's told:
That's not the point. You remember-his face, what you did wrong, what happened to make you lose him, and, next time, you do a kid better, and you don't lose him.
Kakashi feels almost compelled to point out multiple shots some down, ask why 'better' does not seem to be good enough. Iruka has that sickly sideways look of pain in his eyes, and Kakashi tactfully does not speak his though, or ask why Iruka drank a shot for Sasuke too.