I used to have a dog, her name was Zöe, named for one of the characters on Firefly.

She was the best dog. Completely safe around children and the elderly, never barked, never nipped or bit, she was smart, observant, and just the biggest sweetheart.

But if she determined that you were disliked by Her People, you weren’t safe.

My wife told me a story about Zöe, that happened while I was at work. We used to live by a river - it was literally in our back yard. A couple of guys showed up, ostensibly to ask if they could go fishing but more likely to look for things they could steal, as the river was maybe 6" deep at that time, and there were much more accessible places to get to the river. My wife wasn’t comfortable with them there, but they weren’t taking No for an answer. Our other dog was bouncing and barking from the back yard, and they were worried about her, and as a result, they didn’t see Zöe, until she started this angry hateful growl that was accompanied by bared teeth an inch long and a 4" wide strip of fur standing up along her back. My wife said “I can’t really control that dog, you really need to leave.” And we never saw them again.

Tl;dr: Your dog is very likely only nice to YOU and people you are comfortable with.