Why did Thornton tell Buck to jump over the chasm?

On one occasion while the men are encamped near the edge of a cliff, John spontaneously says to Buck "Jump," at which the dog without giving a second thought bounds to the edge of the cliff, preparing to leap off of it. Thornton grabs hold of him, holding him back with help from Hans and Pete, marveling at the trust and loyalty Buck has given to him. He would leap to his death for this man without any hesitation!