"I will if you guys start having trouble because of me," Sam said sternly. "Castiel has done a lot for you. He fell for you, and --" Sam drew in a heavy breath and sighed. "Just don't screw that up, okay?" He looked down at his beer again. "You need to keep it all together. Don't distance yourself from him. Be happy and whatever."
Maybe she was out of practice with the relationship stuff, being a demon and all, but it was common knowledge that stuff like this didn't exactly end well for people who kept secrets from each other. He wasn't the master at advice on the dating stuff on account of his multiple failures of the past, but even Sam knew that.