Pain is a permission slip. You hurt me, I have permission to hurt you
I believe that pain is a message that your body is in danger or being damaged. I don't think one should attach any more philosophical meaning to it.
Ethically I don't think that being attacked (even in a painful way) changes the situation any. Either way you need to defend yourself (so I'm not advocating letting somebody beat you while you stand there). Responding to hurt with hurt only perpetuates the situtation.
Just so somebody doesn't jump on me for promoting Aiki-dance instead of 'do' I think that if you're in a dojo and you're attacking your partner so they can learn a technique, an attack which matches their level of experience is very appropriate (and even white belts should see full attacks so they learn not to freeze when it happens.
That's my $0.02 for the moment... back to the thesis...