What reason does the stranger on the train give the conductor for Buck behaving the way he is on the trip? Why does he said this?

Trouble is brewing in California. Gold has been discovered in the Arctic, and everyone is rushing to the Northland, looking to cash in on the find. Men are looking for big dogs strong enough to toil endlessly and withstand the bitter cold. Enter Buck, one such dog, and the main character of Jack London's tale. Buck lives in the home of Judge Miller in Santa Clara Valley. The house is large and spacious, and the grounds are beautifully laid out with orchards and gardens. For his entire life, Buck has lived here in splendor. Other dogs are present, but Buck is above them all. Buck is the king of his domain, believing himself the benign master of everyone, including his humans. He is neither a house-dog nor a kennel dog but goes where he wishes. The whole realm belongs to him. He escorts the Judge's daughters on walks; he hunts with his sons, carries his grandchildren on his back. A mix of St. Bernard and Scotch Shepard, Buck weighs only 140 pounds. He is not as large as his father was, but he carries himself like a king. Hunting and walking keep him fit, and he rejoices in sport and play.

once upon a midnight dreary, while i pondered, weak and weary,

explanation:

what is the context here? we need to know what story you're talking about : )