When John Thornton states, “It is splendid, and it is terrible, too,” what is he referring to?

Thornton marvels at Buck's loyalty and devotion to him. On a whim, he instructs Buck to jump off a cliff. Buck does and the men have to haul Buck back to safety. Thornton thinks this devotion is splendid but terrible in that Buck will override his own instincts to obey.