There's a million and one reasons , its not always black and white. From educated stuff that knows the craic to tiny vixens that will squeeze 20ft up a rabbit tube to get away from a dog. If you are saying you get everything, every time. Then you're the first terrier man Ive known to say it.depends if your doing it for sport or for pest control. I would respect the foxes efforts and give it best if it managed to squeeze 20ft up a tight rabbit tube were the dog hadn't a chance of fitting.I'm just giving examples.