Decision is final
Although Katie apologized and said that it would never happen again, John had enough. He realized that this kind of behavior was happening all the way from the start of their relationship and she was not going to change anytime soon. He needed to represent himself and look after his own feelings.
A lot of people gave John a hard time with this choice, but it was his decision to make in the end. We guess that the man just really loved his dog and didn’t want anyone to injure it. But we think the dog was just a preview of what was in store for their relationship in the future.