It's extremely obvious that this is not a subject that Dean wants to talk about. And really, Chuck doesn't think he can blame him too much. Because leviathans are not something to be taken lightly, they aren't something to just shrug off and move on with. They are dangerous and they need to dealt with carefully.

Although, he supposes this is when the Word of God tablets come in. Things are finally beginning to make sense. Chuck gives a slow nod, taking a few seconds to fully absorb this information and putting the pieces together.

The next logical question, of course, is, "Why... why open a portal to Purgatory?" Because Purgatory was a locked door for a reason. And if this someone is trying to get to Purgatory they are either really stupid or really desperate.