This is so weird (and wonderful) that I had to share. When Sidiqi reached about one year of age, he began to snap at other dogs at the dog park, where he had been visiting without problem since he was four months old. He was never aggressive, just inappropriately defensive. In other words, another dog would bound up to him, and he would snap defensively even if it was obvious that the dog just wanted to play. He seemed to think that he always had to protect himself.
The it escalated. We would take him for walks, and his first reaction when he saw a new dog would be to growl and strain at the leash. I always felt this was fear-based, and I blamed myself in part. When he was little, he would race into the dog park, which would set off the chasing instincts of all the other dogs there. Invariably, he would end up with 6 dogs after him, and eventually they would catch up and roll him. When it got hairy, I would wade in and save him, which I think may have made him more afraid than he would've been.
Anyway, we didn't stop taking him places, but I stayed right with him. At the dog park, he became more interested in smelling the grass in remote corners than he was in playing with other dogs. I didn't leave his side, and whenever he did snap at another dog when I felt his defensiveness was unwarranted, I took him off to the side and made him sit and stay. When we took him to town, I had a pocketful of treats. Every time we saw a dog, I would give him a treat. Trying to build good associations.
It didn't seem like this was working. He never attacked another dog, but he was still weird about them. a couple months ago, we visited my mother, who has a huge, goofy lab. I'm convinced her lab has magical powers because Sidiqi hasn't growled at another dog since. He has fun playing at the Bark Park again (after he spends 10-15 minutes just enjoying the smells by himself).
In truth, I think our consistency in working with him made a difference... it just took a long, long time. For those of you who are in the midst of it, stick with it! There is hope. :)