Why does Buck dream of an ancient man?

What could that relationship offer him that is missing from his other relationships with men

Buck's dream symbolizes the equality between the animal and the man in ancient times.

Explanation:

"Buck's mind is taken over once again by the hairy, primitive man of the ancient world. Buck's desires become clearer along with his memories. he recalls that "the salient thing seemed fear."

This was a simpler primal time when dog and man were closer in both in purpose and intellect.