Why did Buck often leave John Thornton's fire and run off into the woods?

Buck nightly hears an ancient call deep in the forest. Sometimes he searches for it. Sometimes he hears it, springs from his place and runs wild through the forest. One night he hears it more clearly than ever before. He goes into the forest and he sees a timber wolf. Wishing to make friends, Buck approaches; but the wolf is afraid of this beast three times his size, and he flees. Several times this meeting is repeated, until finally the wolf understands Buck's intentions, and they sniff noses.

The wolf motions for Buck to come with him, and Buck is wildly happy as they run. He now understands what the call meant, running wild and free with his brothers. Suddenly, Buck remembers John Thornton. He turns back, and for an hour the wolf runs with him, confused, but finally Buck must go alone. When he enters camp he overwhelms John Thornton, so happy is he to see him. For two days he does not leave the camp or let John Thornton out of his sight. Then he once again becomes restless. Buck sleeps away from the camp, wandering the forest and searching for his wolf brother for days a time. He kills a black bear in a vicious fight, and when he returns to the kill and sees a pack of wolverines, he kills two of them as well. The urge to hunt and kill grows stronger in him.