At least they could both bask in their guilt together. Dean was rather certain that his own guilt was part of the reason he felt such an overwhelming need to pay Sam back properly for what happened last night. His brother might have gotten a lot of out that, but Sam did most of the work, while Dean just held on and took it. He felt it was only fair to give Sam this.
Only hearing Sam saying something but not actually making out the words properly, Dean slid his free hand along Sam's thigh, bobbing his head as he sucked on Sam's length, deciding his jaw hadn't ever ached this much without him being punched in the face.