"There's nothing wrong with wanting to do something. Hell, I don't want to just sit around either." Really, he didn't. And he didn't want Seamus to think it was a bad decision all together. Just one that was poorly executed. Not to mention, at the end of it all, it made Dean feel a bit unwanted in a way that he couldn't do much. There was still plenty he could do! Just not anything magic based.
And as much as he didn't want to sit around, Dean hated the fact that he was being told to run when a situation got hairy. He had spent the last year doing just that, and physically showed that he didn't like the idea.
So, he found himself glancing away. It was what most people wanted him to do. Dean knew that, but couldn't agree that he felt the same way. "Only if you tell me that you're not making this deal because you think I can't take care of myself without magic now," he remarked as he held in a slight grumble. Of course, Dean appreciated that Seamus was looking out for him. A living best friend is better than the opposite. He couldn't deny that the subject bothered him for quite a while.