The Call of the Wild by Jack London tells the story of Buck, a large dog who lives a life of ease on Judge Miller's estate in the Santa Clara Valley. The discovery of gold in the Canadian Klondike creates a demand for strong dogs who can pull sleds, and Buck is kidnapped and sold to ruthless dog traders. Suddenly thrust into a dangerous situation far from home, Buck has to learn to survive. Slowly, over the course of the tale, Buck becomes powerful, cunning, and accustomed to the ways of the north. At the same time, he begins hearing the primordial call to return to the wild of his wolfish ancestors.

By chapter seven of the book, the only thing that keeps him from running off into the forest to answer the call of the wild is Buck's deep love for John Thornton. Despite his love for Thornton, however, the wild calls Buck in a visceral way: "It filled him with a great unrest and strange desires." He sometimes gets up, leaves the campfire, and runs for hours through the forest filled with a strange joy.

When the wolf appears at the camp, Buck is intent on befriending it because it represents a joining or bonding with a brother from the wild. It gives him a sense of belonging and helps him make the full transition from domestic pet to savage forest beast. While searching to reunite with the wolf he met, Buck learns the ways of the wild. Although he continues to return to Thornton, by the time Thornton is killed, Buck is prepared to forsake his relationship with humanity and take his place as the leader of the wild wolf pack.

The reason for this is that, by this time, Buck is really hearing the call of the wild very strongly.  It is only his love for Thornton that keeps him from following the call completely.

So the reason that Buck wants to hang out with the wolf is that they wolf represents what Buck really wants to be.  He is like a dog, but he is wild and free.  He is the way dogs used to be (in the visions that Buck has of the past).

So this is really a way of showing that Buck is getting so close to just becoming wild (as he will when Thornton gets killed).