Dean and Sam in “Unfinished Business”

Dean

He said the river ends at the source.

Sam

What does that mean?

Dean

Maybe nothin'. It was the last thing he said before you guys busted in.

Sam

Dean, look man we had to...

Dean

Hey, no I get it, alright. I was going to kill him, and I couldn't stop myself.

Sam

We'll figure it out alright. Remember what Cas said about needing a powerful force?

Dean

Yeah so?

Sam

So, I've been thinking, look. Cain still has the Mark right? And he's lived with it, for years he's lived with it. So yeah, the Mark is strong, but Dean maybe there's a part of you that wants to give into it, and maybe you have to fight that, you know? Maybe part of that powerful force has to be you.