What test did Buck perform for John Thornton, and why was Thornton afraid?

Thornton loved Buck but decided to test his loyalty. Thornton was sitting with some others "on the crest of a cliff which fell away, straight down, to naked bed-rock three hundred feet below". Thornton decided to command Buck to jump off the cliff. Buck was going to until Thornton pulled him back at the last moment. Thornton was afraid that Buck was about to jump to his death.