Sam: Cass, you know why you can't go with us, right?
Castiel: My angelic presence will be sensed by Michael thereby nullifying our hopes of a sneak attack.
Sam: Yeah, sorry.
Castiel: And you need me to stay here to babysit Nick and Jack.
Sam: It's not babysitting, Cass.
Castiel: Only in the sense that their both not infants but they both need to be supervised. Jack is lost without his grace and Nick is...he's just a mess.