Superman feels things...that is ridiculous. You don't only react when things cause you pain I am sorry that just isn't how physiology works. (or emotionality...he could also be reacting to how people are reacting to him in that moment) Especially from behind he might react involuntarily.
Like I said 5 year olds used to punch me all the time. Never once did it hurt or even sting. (it usually didnt regsiter unless they went low

) Sometimes I still blinked. Does that mean I am not an adult Black Belt?
Hell on the old Superman show he used to duck when people threw stuff at him