Somewhere, in a dark room, Bush strategists are sweating over how to prevent their man from giving "those expressions" again in the next two debates. A keen observer of Bush will notice that it's how he behaves in the public arena -- he closes his mouth, cocks his head, and looks as though he is dominated by the internal emotion of the moment. Lord knows we all have "those expressions" when our facial muscles are uncontrollable due to the extreme sentiments we cannot shake off.
Those up-and-down emotional swings are the result of, often, not knowing how to behave in a humiliating situation. On the night of the first debate, Bush realized that he was being humiliated, in public and on camera, for having committed something terribly wrong, and then the humiliation overwhelmed his ability to act "normal". Why didn't those emotional swings happen to Kerry? Simple, Kerry hasn't done anything embarrassing. Psychology 101; any college student can tell you that.