My first Newf (when I was a kid) was very friendly to people but uncharacteristically nasty to all other dogs. It all started when my dad was taking him - Bruno - for a walk in the park when he was around 7 months old, and he was attacked by a full grown German Shepherd. It was a bad fight, and Bruno's face was a bit cut up. But the Shepherd was almost killed.
Every since that day, when he tasted blood at a very early age, he just didn't tolerate being around other canines. He even growled at stone statues of dogs.
He died at around 12 (the age every single one of my dogs has died), and it was an incredible event. He spent about 3 days digging a huge hole in the corner of our property, and when he was done he simply layed down in there and died peacefully.