Xenophon Cyropaedia 8.3.49

[49]
  Now, Pheraulas was naturally a good fellow, ¹ and nothing seemed to him so pleasant or so useful as to serve other people. For he held man to be the best and most grateful of all creatures, since he saw that when people are praised by any one they are very glad to praise him in turn; and when any one does them a favour, they try to do him one in return; when they recognize that any one is kindly disposed toward them they return his good-will; and when they know that any one loves them they cannot dislike him; and he noticed especially that they strive more earnestly than any other creature to return the loving care of parents both during their parents’ lifetime and after their death; whereas all other creatures, he knew, were both more thankless and more unfeeling than man.