After meeting John Thornton; what was Buck's greatest fear?

Buck's biggest fear was that John Thornton would leave him.

For a long time after his rescue, Buck did not like Thornton to get out of his sight. From the moment he left the tent to when he entered it again, Buck would follow at his heels. His transient masters since he had come into the Northland had bred in him a fear that no master could be permanent. He was afraid that Thornton would pass out of his life as Perrault and Francois and the Scotch half-breed had passed out. Even in the night, in his dreams, he was haunted by this fear. At such times he would shake off sleep and creep through the chill to the flap of the tent, where he would stand and listen to the sound of his master’s breathing.