Sam naturally demands to know how Dean knew Ruby was a demon just by looking at her. Dean doesn’t exactly mean to lie; he’s busy enough trying to keep the details of his work in Hell from tumbling out of him. Sam’s been hanging out with demons and Sam’s keeping secrets. So if Dean says “I felt her,” that’s pretty much the same as “I heard her thinking,” right?
Now Dean’s back but things between them are strained, not least because of the fucking angel over/on Dean’s shoulder. Dean doesn’t need Castiel to tell him to keep an eye on his own brother, thanks very much, but Sam’s sneaking around now and Dean doesn’t know how to deal with that at all. Before, when Sam ran away from home and then when Sam went to Stanford, Dean hadn’t had any advance notice that Sam had plans. That might’ve been better: the dread of knowing that Sam was just waiting for Dean to turn his back was a new kind of awful.
Sam doesn’t forgive him for killing Ruby when she comes back. Dean tells himself it’s because of the blood, because if Sam had feelings for a demon, for someone who’d gone to Hell a person and come back out because she got all the way through the system, then Dean—
Dean tells himself it’s because of the blood. He follows Sam to his new contact and hears the demon say out loud that Ruby wasn’t the only one in their group fighting Lilith while in his head it’s all “can’t believe I have to do this shit, fuck Lucifer anyway.” He watches Sam drink.
Later, there’s yelling, but nothing’s resolved. Sam doesn’t believe Dean that he’s part of the devil’s plot, because he thinks that he’s getting stronger and that he’s in charge. Dean can’t deny the first and doesn’t know how to deal with the second.
They live out of the same car, but they’re not together. Sam goes and drinks whenever he likes; the demons he meet don’t know anything beyond their own orders, and the names in their heads are useless. The seals break, one by one. Dean doesn’t understand why he was brought back for this, like thirty years on the rack and ten off wasn’t enough.
When Castiel tells him to use his best skills on Alastair, Dean doesn’t even have to try: Alastair doesn’t know anything useful. His mind shouts louder than all his slithery words. Of course, Uriel doesn’t believe the monkey-brain, and Castiel won’t help him. So Dean picks up the knife like they want him to, and that goes about as well as he could’ve expected. In the hospital after, Castiel tells him that he was the first seal.
After Sam breaks out of the panic room and beats the crap out of Dean, Dean almost says fuck it. Could Lucifer do a worse job with the world? But Lucifer’s made Sam his junkie, and Dean can’t let that go.
He gets to the convent just in time to see the door slam between him and Sam, Lilith, and the demon of the week. Then there’s Sam’s voice, a stream of “No no no” so shocked and horrified that the demon’s gloating is barely intelligible.
Dean breaks through the door because that’s what he’s here to do. He kills the demon because it doesn’t get to celebrate fooling Sam, and he grabs his brother because he carried Sam into this, nearly thirty years back, and he’s going to carry Sam out.
But instead of obliterating them, the flash of light leaves them on a plane, somehow. Sam doesn’t ask why tears are running down Dean’s face. If he could spare any attention for Dean's breakdown in the midst of this clusterfuck, he'd think he knew why, but he’d be wrong.
After all, he’s not the one who just started to read his brother’s mind.