Put the author's conclusion into your own words:

not every dog who tries to bring up a man is as successful as my setter. the answer lies in understanding. the dog must be patient, and not work himself into a tantrum if his man cannot learn to chase rabbits or wriggle under fences as well as the dog does. after all, as my setter says, "it's hard to teach an old man new tricks."

The author means that his dog owns him and tries to raise a human being, thus reversing the typical relation in which a man owns a dog. Therefore, he claims that the dog must be understanding and tolerant when he trains his man and he is not able to perform as well as he does. Finally, he concludes that it is arduous to make an old person change his or her habits.

merry christmas to you, too. may god be with you all in doing your work and just living a great life in general! < 3