My previous 4 Mastiffs were all completely mellow and relaxed around other dogs; they would never pick a fight and were social to a degree. This Mastiff is an American Mastiff, which means he is 1/3 Anatolian and 2/3 English. He's a totally different ballgame -- a real working Mastiff with strong LGD traits.
I didn't raise him any different than the others; in fact, I probably improved my approach given that I am more experienced.
At 1 year old, a switch flipped and he became dog aggressive. He doesn't give any warning, and when he goes, he goes. He is in it to win it, and will kill the other dog if not stopped.
The trainers, behaviorists and my vet all agree: he cannot be near other dogs, ever. It sucks but it is what it is.