Why was white fang not demonstrative

From the text:

"He was too old, too firmly moulded, to become adept at expressing himself in new ways. He was too self-possessed, too strongly poised in his isolation. Too long had he cultivated, reticence, aloofness, and moroseness."

In other words, you can't teach and old dog new tricks. White Fang was unable to display affection because he'd never learned how; he'd never barked, never been the silly, loved dog. Thus he worshiped his master in silence.