"I think Cas's got a point," Sam pipes up, concerned. "I mean, they'll go after all of us first. And they know that we're each other's weak points; you can't say that if me or Cas get nabbed, you won't go charging straight in after either of us." It's given that if Dean were to get snatched that Sam would also barrel after him. Cas- maybe, but then again the Leviathan, with all of Castiel's memories, probably know that as well.