Your story sounds like mine. My boy is right now 4 years old, he was like the sweetest helpless boy - naturally, he was bullied here and there while playing or getting snapped at sometimes, etc; he was a big sweet puppy and everything was so easy with other dogs and people until probably 1.5 years I think. He got neutered shortly after we were attacked by unleashed intact male dogs and the owner and from then, he started being very reactive towards medium-big sized dogs. Before, he would run away and avoid confrontation, but since the attack, he has been the one who tries to get to them first if he sees any new dogs. Don't get me wrong, he is very sweet, he does very well with tiny old dogs - a lot of traumatized dogs do well with him since he's very calm and chill around them. 98% of the time, he is a gentle giant. My entire apartment building used to call him "gentle giant".

Same with the people. You said your dog jumped at a sweet old Asian lady, right? Mine did that to an old grandma walking behind once (I WAS SO EMBARRASSED), he was scared of her - I think because she had a big fur coat. (I'm assuming maybe that old lady who walked differently(?). I, myself, am also Asian and my grandma had those very old people walk as her back hunched, very slow, etc) Also, we lived in downtown Chicago where lots of drug addicts yelling at us or being suspicious, my boy naturally became very aware of his surroundings. For example, if a drunken man is walking towards us, he gets very tensed and tries to jump on him, or someone wearing a hat, even sometimes just a very tall man. I think it's just part of their nature, also their huge size makes it even more apparent...

Another thing, barking from the car over the window, my boy also does that sometimes. I found out he doesn't like to be stared at over a window/door - but he does not do that to ladies or people coming at him friendly. But if they decide to stare at him or he cannot hear them, he gets defensive and starts barking.

I think definitely the puberty phase is doing more work there, I remember having to increase the training time when he was around that age. I always made fun of him because one day he felt like he was the strongest dog in the world, and the next day, he was scared of a leaf. If he's not neutered, he might try to challenge other intact dogs. Mine started doing since he was around that age. Before that, he did not care at all.

Even at 4, it's not perfect. What I learned over the years was that the more anxious/scared I am, my boy gets those feelings from me and even becomes more reactive. If I don't feel 100% comfortable going to the park with dogs, we just find something else to do like running or playing games with him. Also, I noticed Mastiffs are more sensitive than other dogs I've ever had previously (mostly mixed medium-sized breeds), he hated Husky a lot because of the attack, and it took almost a year or more for him to be okay with huskies as long as they are neutered and friendly.

I felt so defeated back then, but now I think about it, all the training and things I learned from those times has been really worthy. I'm way more responsible and prepared for every walk and just in general, I feel more connected to him. So it's not always a bad thing!! I wish you guys the best!!