How does Buck displays his affection for Thornton?

This is similar to your last question. Buck loves him because he shows his need for Buck, repeatedly demonstrating that Buck can help him in ways that others can't. When Buck wins him 1600 dollars or saves him from a deadly rapid, John Thornton is honoring Buck, honoring his power and his loyalty. This loyalty goes beyond the loyalty of the team. Buck depended on those dogs for his life, but he depends on John Thornton for his happiness.