Ah, the life of leisure. Like most initial situations, this is rather boring and unexciting, and begs for Something Fun to happen next.
Something Fun happens. Or rather, something illegal and kind of sad. We identify this as a conflict based on the clubbings, illicitness, chains, and all around sinking feeling that a story is about to unfold.
Spitz stands in the way of Buck completing his transformation, so Buck does the only thing a protagonist can do when faced with a complication: he beats the heck out of it.
Sled: crash. Bystanders: oh no. Reader: gasp. Sounds like a climax to us.
John Thornton adds an element of tension to the end of this story. Once Thornton almost dies we begin to fear that he actually will die. Whether or not Thornton will die constitutes the suspense of Call of the Wild.
This is told in summary, to make it even less suspenseful and more denouement-y. Buck kissing civilization good-bye for a life in the wild was sort of inevitable anyway, so we can relax once we get here.
Just the kind of conclusion we would expect from a book with such a title as this.