Mirror
They're perfectly crafted liars. It's critical in this life; you've got to lie, and you've got to be good at it. You've got to be good enough to make people believe the sky's green if you wanted them to.
But they've never been able to lie to each other, which is why it's at once so easy to spot and difficult to understand. Dean used to be able to read Sam like an open book, but the person he's come back to is closed-off, locked and duct-taped shut. The same book, written in a language he can't read.
It's easy to believe, sometimes, that nothing's changed, because they do the same things they did before. They rhythm's the same, even if the tempo's changed, even if the beat's picked up speed. But it's just a mask now, only mirroring what it was before.
Which, to be perfectly honest, he expected. But that's the one thing about mirrors; they can't lie. The image can be distorted and reversed, but what's staring back at you is always truth in it's sharpest form. Pretending they can ever be anything like they were again is just the easiest way to avoid looking in the mirror.