My take on Merkel's expression was entirely different, and here's why.
In my family, we do a lot of shoulder rubbing, call it a family tradition, (it's probably because we all carry our "stress" in our shoulder muscles.)
Anyhoo, if I were to rub my sister's shoulders, like Bush did to Merkel, she would make exactly the same gesture...why...because it hurts her, and she reacts just like Merkel, raises her shoulders, throws her hands up and then pushes the "shoulder rubber" away.
When I saw the photos, that's immediately what I thought of, the poor woman must have had a bad shoulder ache (perhaps mentioned it to the Prez) and when he grabbed her shoulders and started rubbing, it hurt like heck, so she reacted to the pain.
May not have been the case at all, but that's what I thought of when I saw it.
Regardless of how we might do shoulder rubbing in our families, and regardless of how the Clinton family discusses boxers or briefs, that wasn't the setting for this gesture. Decorum is not mere a nicety.