There are many good things to say about owning a large dog and letting that dog serve as a watchdog. His or her growling and barking are likely to be louder and more menacing to a would-be burglar. Plus the dog may well be able to defend himself when necessary. Now I hasten to add that large dogs are just as likely to be affectionate to the point of greeting a burglar with a wag of the tail and allow that burglar to do as he or she pleases in the house. You don't want a dog that's too friendly to strangers.
On the other hand, a smaller dog may be perfect! It may have a shrill bark and be less likely to be intimidated by a burglar or other criminal. And it can easily bite and nip as well. A smaller dog also tends to eat and poo less and may be easier to walk and control on a leash. At the same time, a little dog is just as likely to be friendly to strangers and allow strangers as burglars to pet and give them treats. So again, you've got to be careful about the type of dog you choose to serve as a watchdog for your home!
Life is hard
3 years ago