When two people quarrel, if we take part with, and entirely adopt the resentment of one of them, it is impossible that we should enter into that of the other. Our sympathy with the person whose motives we go along with, and whom therefore, we look upon as in the right, cannot but harden us against all fellow-feeling with the other, whom we necessarily regard as in the wrong.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“Secondly, I say, That wherever the conduct of the agent appears to have been entirely directed by motives and affections which we thoroughly enter into and approve of, we can have no sort of sympathy with the resentment of the sufferer, how great soever the mischief which may have been done to him. When two people quarrel, if we take part with, and entirely adopt the resentment of one of them, it is impossible that we should enter into that of the other. Our sympathy with the person whose motives we go along with, and whom therefore, we look upon as in the right, cannot but harden us against all fellow-feeling with the other, whom we necessarily regard as in the wrong. Whatever this last, therefore, may have suffered, while it is no more than what we ourselves should have wished him to suffer, while it is no more than what our own sympathetic indignation would have prompted us to inflict upon him, it cannot either displease or provoke us.” source