While not all people may do this, there are definitely some who will approach any dog they see while they are with their dog just because theirs is friendly. Unfortunately, that can lead to fights between the two or more dogs due to their lack of consideration that the other dog could be aggressive.
Now, before I go into detail about this subject, I would like to say that I am not intending to judge those who do this. In fact, I used to be one of the people who only thought about the fact that my dog was friendly, and the only reason I found out about how it can affect others is that I ended up adopting a dog named Zoey, whom I found out afterward was aggressive towards other dogs.
My personal experience with my aggressive dog, Zoey, wasn't as bad as others may have experienced. She was an angel around people, but she struggled a little more around fellow canines. Thankfully after carefully introducing her to my family's other dog, Salty, they got along fine, although she definitely considered herself to be the dominant dog.
The biggest problem was on the walks. I would try to walk her every day, and although I live in a more forested area than a complete neighborhood, I would still often pass by other dogs. If they were behind a fence Zoey would pull and maybe bark, but not as much as when they were just across the street, or sometimes right in front of me. I often times would have to quickly cross the street, or sometimes even cut a walk short so that she wouldn't get aggressive and possibly hurt any dogs because the people were having no problem approaching me while walking in the middle of the road.
There were admittedly a few times where Zoey attacked other dogs, but the times it did happen, the dogs were on our property. We didn't have a fence on our property, but we also lived on 6 acres of forest, and our dogs were good about staying on our property. Because of this, we usually didn't have to worry about my dog while at home. The first dog she attacked was walked down to our house by people we knew, and the dog, named Roary, was known to be a generally nice dog, but Zoey was one of the dogs he would occasionally show some aggression towards, whether she showed it first or not. He had just gotten stitches from a surgery when he was brought down to our property, and Zoey was outside sunbathing while I was doing homework.
I didn't notice that Roary was there at first, but then I noticed Zoey was sniffing him while her hackles were up, but she hadn't done anything aggressive yet. I felt nervous so I started to go outside when I heard barking. I quickly rushed outside to see that Roary's stitches had been opened, most likely from Zoey and not from simple movements. Nobody was sure which dog started the fight, but we ended up driving Roary to the animal hospital and fully paid for the medical attention needed. He ended up being fine and mostly just needing to be restitched.
The second time was a very similar situation. I was at work when it happened, and I'm not sure which dog it was, but what I do know is that the dog was brought down to our property while Zoey was outside. It once again resulted in us bringing the dog to the animal hospital and fully paying for the medical expenses, and thankfully that dog ended up okay as well.
Unfortunately, after that, it was decided that it was becoming quite difficult to take care of Zoey due to her aggression, as well as the fact that it would have been much too stressful for my parents to take care of her once I began attending college. Because of this, I ended up having to give her away on October 5th, 2016, and I can still remember every detail of my last minutes with her before heading off to school.
When someone first sees a dog, the first thing that often comes to mind is "Awww it's so cute," and often times that can distract you from other aspects, such as subtle signs of aggression. Some that I would recommend looking for is dilated pupils, panting when it isn't very warm out, its tail is down or in between its legs, its hackles are up, even if it's just slightly, and/or its teeth are showing. These are just the behaviors I know of, but I'm sure there are many more that I have yet to learn and I would recommend looking into, most of which are caused by fear.
In the end, not every dog is friendly. No matter how lovable your dog might be, always keep in mind that the dog walking across the street from you might be aggressive towards other dogs, and make sure it's okay with the owner before approaching. I'm not saying to fear every dog, I'm just saying that if a dog is showing any sign of aggression, or the owner looks nervous and/or tense, then please give them space unless they say otherwise.
