In The Call of the Wild, why was Buck ready to obey the calls from the wild?
The end of the book gives us the primary reason why Buck reverts to his primal nature, and that is that his human companion John Thornton is dead. There is no longer any "master" over Buck's wild side. He had been the lead dog in the sled pack for some time, and it was only natural that he revert to the wild side as a leader of the wolf pack.
Buck had been allowed to explore the "wild" side of his nature toward the end of the book as John grew to have a more companionable relationship with Buck. When Buck came back from a romp to find John dead, there was no longer any reason to be a "domesticated" dog
Over the course of the book, Buck is getting more and more ready to obey the call of the wild. Finally, at the end of the book, he does obey the call and becomes the leader of the wolves.
In the book, Buck gets more and more ready to obey as he gets to be more and more of a wild animal. All the lessons he learns about survival make him less and less "civilized." In Chapter 7, after John Thornton is killed, there is nothing left to tie Buck to civilization. At that point he is completely ready to obey the call of the wild.