My dobie/lab mix was the sweetest love bug you could imagine. He never hurt/bit/nipped at anyone, nor did he ever display any signs of aggression other than some alerting (which I don't consider aggression, but his bark went down 3 octaves haha!). Until...
He was old enough to discern how uncomfortable our druggie neighbor made me. The guy would always knock on our door when my father wasn't home, especially when I was in the shower. No joke, I began taking a sword with me in the shower just in case he broke in until Dodger was older. His alert/guard bark was incredibly deep considering his size, and it was enough for the neighbor to leave. On more than one occasion, when I'd walk Dodger at night, he'd be his usual bouncy self until our neighbor came walking down the street. Even with a friendly, "Hi Dodger, remember me?" my dog was not having it. He'd stiffen, hackles raised, tail up, mouth closed and give a deep growl with hard eyes that followed my neighbor out of sight. He was 50lbs but he sounded like a 150lb monster. And I'd stand there, give a curt smile and a nod as he passed while also whispering "good boy, what a good boy" and patting Dodge as if I was trying to "calm him down." I miss that dog. He's now my grandmother's guardian as she lives in a house alone in AZ. She says he's such a great protector and that she can't live without him, he's just so loving and lovable.
Epilogue to this novel I wrote lol: That same neighbor was found dead on our back lawn a year later by mom and sister. He'd been the reason our AC unit was breaking down multiple times a year, costing my parents hundreds and hundreds. He was huffing the freon from our unit and multiple units around the neighborhood when he couldn't get his usual hit.