It wasn't a good thing when she up and vanished on him, he wanted to point out. Ruby had no idea how difficult life had been for him without the tiniest hint of blood to hold him over. Personally, Sam would have been far more pleased with everything if there were other demons with similar blood that he could take away. Then he'd be able to drink without always having to rely on Ruby. It couldn't have been a pleasurable experience, having a giant, hungry, hunter clinging to her arm every time he needed a fix.
Plus, he could tell, especially after that last round. It was hurting her more than she'd like to admit. Sam didn't like that at all.
He looked uncertain of her response. She didn't seem fine. Not as strong as usual, but not as bad as how she was when Alastair handed her back ages ago. Now that had certainly been something worth worrying over.
"It's okay," Sam agreed, "tell him what you need to, Ruby. Whatever makes it easier." It would. Castiel wouldn't have to wonder why she came to him first rather than the one that she loved. It was the more logical approach.
"I..." He leaned back, sighing. "I won't take that much from you again, Ruby. I'm sorry." And he was. Mostly. He sure wasn't sorry about the way it was making him feel.