In this particular context, it was a little disturbing to realize that Dean could read him that well. It meant that Dean as a demon would probably also be able to read Sam like that. There was usually a certain level of stupidity on a demon's part, because they underestimated human ability by comparison to their own power. Dean, on the other hand, had an extremely accurate grasp of what Sam was capable of, and how he worked.
Warily, he reached out for the holy water, as if half expecting it to be a trick. When it wasn't, he turned it over in his hands for a few moments, contemplatively. He ran his fingertips over the top of the bottle, but didn't open it. Yet.
"Next thing we try," he said, after a moment, "Or the next thing you try?" It wasn't an accusation, and it wasn't a request to be included. He wasn't going to beg to be a part of this, and he wasn't going to yell, even if Dean chose the latter. Sam had been yelled at for making decisions like this, and that had only served to alienate him even further.
It was just a point of clarification. He wanted to know where they stood, so that he could figure out what to do next. But there was an edge to it, too: could even be considered a warning. That if Dean was going to do whatever he liked with no regard for whether Sam should be included, then Sam would have no problem doing the same.