If a dog and cat are strangers, they will fight each other. If they are friends, fighting never enters their mind. Why is this?

I had an Australian shepherd who was raised with a cat. They were together from six weeks old. They were best friends. We eventually brought two new kittens, six week old siblings, into the family. To the dog, the kittens were no big deal. He didn’t seem to care or notice them. Of course I took time with both kittens to introduce them to our dog. One became fast friends with him. The other would stalk and hiss at him years later.

Saying all that, I think it is one part that dogs are territorial and protective of their domain. Another part is that perhaps your dog sensed the strange cats’ unease and automatically went into defense mode.

Yes, it’s mostly a territorial thing and, for many cats, a fear of predation too.

My old cat that died a year ago was extremely upset with strange dogs, even though he was fine with mine over the years. He, on several occasions, flat out jumped on and attacked dogs that came into his yard and he scared the crap out of them. One was a Rottweiler and literally leapt onto it’s shoulders and rode it shrieking up the driveway.
It’s a two way street. haha

Pack mentality.

Your dog views your cat at home as a family member, part of it’s pack. The cat it see’s on the street is just a wild animal for it to chase.

When I got my cat I slowly integrated/introduced him to my dog (a full grown siberian husky, very high prey drive). I would keep them in separate rooms and rotate them every few hours so they could get used to each others scents. Eventually I supervised a few introductions, and then let them be in the same room with me, and now I could leave them alone for hours and it would be fine. Hell the cat probably annoys the dog if anything, but save for some crazy licking the dog leaves him alone….. It’s the same reason he goes less crazy when I or my g/f come to the door. He knows us, we’re part of his pack, his family.

It’s probably for the same reason you’re less likely to fight with a friend than with a stranger.

Also, not all animals who live together are friends.