when you slap some one, thinking physics wise, you should not feel sorry. can you think

why?

According to Newton's Thrid law of motion, For Every Action there is an Equal and Opposite reaction.
So if you slap someone , the hand exerts a force on his face which makes him feel pain and also the face exerts back an equal and opposite force on your hand, and you will feel pain in your hand.
Hence actually, you don't have to feel sorry, Cuz.. Its Equal Equal now :-D