That excuse was bullshit, and Sam couldn't believe he was hearing it. He stared at Dean incredulously for a few moments, and then, with a few quick movements, unscrewed the cap of the holy water bottle and threw a good portion of it directly into Dean's face.
It didn't do anything, at least, not that Sam could see. He screwed the top back on and set the bottle aside.
"So," he said, "If that's not demon-you or the Mark talking, then it's just you being a dick." He let that sentence hang for a moment, but he was too angry not to carry on. Now it was obvious in the way he was holding himself, and the way he gestured emphatically between the two of them with his one good hand. "If you hulk out, or die, that will still be a story that I have to carry around. What if it was me, Dean? What if I was the one walking around with that Mark, and I tried to cure it without you there, and something happened? Would that story be better if you found out about it second-hand?"