"Nah. You pranked yourself by never getting used to it." To prove his point, Sirius put the bottle to his lips and tilted his head back, drinking a bit more eagerly than he probably should have. Then he set the bottle back down and shrugged. "You get used to it."

Maybe that was it. He had started to no longer be used to not having it, which was a different problem. "I made a promise, I'm only allowed to empty one of these bottles. So you better be happy that I'm willing to share."