When we see one man assisted, protected, relieved, by another, our sympathy with the joy of the person who receives the benefit serves only to animate our fellow-feeling with his gratitude towards him who bestows it.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“We should be sorry for their sakes if it was destroyed, or even if it was placed at too great a distance from them, and out of the reach of their care and protection, though they should lose nothing by its absence except the pleasure of seeing it. If it is man who has thus been the fortunate instrument of the happiness of his brethren, this is still more peculiarly the case. When we see one man assisted, protected, relieved, by another, our sympathy with the joy of the person who receives the benefit serves only to animate our fellow-feeling with his gratitude towards him who bestows it. When we look upon the person who is the cause of his pleasure with the eyes with which we imagine he must look upon him, his benefactor seems to stand before us in the most engaging and amiable light.” source