"Dude... you and careful don't even go in the same paragraph, let alone the same sentence. You're one of the most reckless guys I know." Okay, so not that Sam knew a lot of people that were still alive, but that was beside the point. Dean was the reckless one, Sam was the cautious one. That's why they worked.
"All I'm saying is, we need to tread carefully. We don't know what we're going to find." Curiosity is what killed the damned cat, Dean. And Sam was pretty sure the Winchesters had had more than their nine lives worth already.