Man, he observes, is naturally much more interested in his own happiness than in that of others, and it is impossible that in his heart he can ever really prefer their prosperity to his own.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“Mandeville considers whatever is done from a sense of propriety, from a regard to what is commendable and praise-worthy, as being done from a love of praise and commendation, or as he calls it from vanity. Man, he observes, is naturally much more interested in his own happiness than in that of others, and it is impossible that in his heart he can ever really prefer their prosperity to his own. Whenever he appears to do so, we may be assured that he imposes upon us, and that he is then acting from the same selfish motives as at all other times.97 Among his other selfish passions, vanity is one of the strongest, and he is always easily flattered and greatly delighted with the applauses of those about him.” source