There will be plenty of time to catch up when they meet back up at their destination. She can hint at it then.

Sam's grateful Castiel saved Dean. But no sooner had the angel appeared, he was gone again. It does work in his favor if Dean can't put together the pieces of where all the bruising came from right away. The last thing they needed is an argument compounded on the existing one. Everything is going to come up eventually. Ruby training him and his powers grown since the last time they talked just seems like the last thing to get around to.

"No, Dean... You're back and that's what I wanted, but I can't ditch her. I won't. It was more than being lonely when you were gone; as crazy as it sounds, we... Ruby and I have something." Unfortunately, it doesn't sound reasonable to Sam. It might be hard for Dean to understand but it's a friendship between them that Sam doesn't know he can find in anyone else. He'd be scared for the longest time to reach out to someone because of what they do. The thing with Ruby isn't Stockholm, it's the first possibility he's seen in years.