Actually, Dean could blame Seamus for the reaction he undoubtedly had. Not that he knew what he said to Harry, but mostly assumed if their shared friend had thanked him for agreeing. He could image what the blonde had exchanged though. If things were reversed, Dean wouldn't have hesitated to add his own girlfriend to the group for safety reasons.
There was a moment where Dean thought that Seamus was going to bail on him. Avoid the conversation all together, and if that was the case, he'd follow right behind.
"Do you even realize how stupid it is to be pissed off because of this?" he asked, only half mirroring how his best friend felt about it all. Dean knew if Seamus found out that he knew the entire time that Lia had been a part of the DA, that it would only fuel the anger. "I don't understand why you're so mad though. You can't protect her from what will evidently will happen, especially with the ways things are headed."