Sam played the argument over in his head. He had tried to make Dean understand but he hadn't wanted to listen. Dean wouldn't even try and understand why he was doing it. Sam was the only one who could stop Lilith. He had to do this, to save everyone, to save Dean.
Wake up and smell the coffee Sam! She's a DEMON! She is using you!
Dean was wrong though, wasn't he? It was Sam who was using Ruby, wasn't it?
"You ready Sammy? We gotta load the nurse and go."
"Yep, coming."
He had to do this. Didn't he?