That was a fair point, and definitely something Sam had been worrying about. It also wasn't anywhere near the point he'd been making. He looked down at the water bottle in his hands for a few more moments, and then raised his head, letting out a breath.
"Look, Dean," he said, "You came in here saying "we" like we're doing this as a team, but we haven't been. You decided to come out in the open and include everyone. We decided to try the soulsword, but you went off and tried it on your own. Maybe you're trying to teach me a lesson about all the times I ran off and tried to fix things on my own, or maybe you think I can't handle this, but I can. And you know, if you're trying to avoid me being worried and hovering around you-- the way to do that is to stop keeping me out of the decisions you're making about the Mark. The less I know, the more worried I am."
It was the truth, and Dean should know that by now. There had already been plenty of times in their lives when Sam had stubbornly, persistently pursued information about something he didn't know or understand, to the point that it had gotten him (and them) into trouble. Up until now, it had primarily been information about himself and his own fate, but now that it was about his brother it wasn't going to be any different. If anything, it just made it all the more important.
"I want to help you. I want to do it on your terms. But I can't do that if you're shutting me out."