If Buck was finally happy living with John Thornton, why did he wander away?

Buck wanders away because he has made friends with a wild Timber Wolf. Basically Buck finally hears "the call of the wild". His transformation from pampered house dog to alpha male wild dog is almost complete. Although he is loyal to John Thornton he is moving away from the ties that bind him to man. After Thornton is killed by the Yeehats, Buck does one last thing for his old master (he gives the Yeehats a major ass-kicking) and then permanently takes his place as king dog of the wild.