What causes Buck to leave his home to become a sled dog?

Buck leads a very happy and fulfilling life on Judge Miller's ranch in California. He has the run of the judge's extensive estate, free to roam as much as he likes. He may be a domesticated animal, but Buck has the kind of freedom that most house pets could only dream of.
Sadly, Buck's happiness isn't set to last. A sneaky gardener on Judge Miller's estate, Manuel, sees an opportunity to make a quick buck (no pun intended). He knows that big, powerful dogs like Buck are much in demand on the Klondike trail. There's serious money to be made out of selling dogs to sled-drivers. So the gardener takes Buck for a walk while the judge is away. But this is no ordinary walk; this is a dognapping. Manuel hands Buck over to a stranger, before tying a rope tightly around Buck's neck. Buck realizes what's happening and attacks the stranger. But he's overpowered, and he soon blacks out. When he comes to, Buck finds himself on a baggage car, heading north on the long, arduous journey to the Klondike trail. His epic adventure has just begun.