I'm sitting here writing this, sobbing like a child (I'm 50). We have two dogs, Red and Kitty (brother and sister) that we adopted when a friend's dog had puppies. We were initially going to only adopt a girl, but Red was so protective of her (making sure she was able to eat, etc...not aggressive protection in any way), that we decided to adopt both. We have had them for almost 6 years now, and they are completely ours - we love them dearly, they're our kids.
At about 1 1/2 years old, we had gone to a family gathering and brought the dogs (it was out of town). They were both fine, tied up with long ties in the back yard with many people around. One friend was outside the fence and stuck her hand through to pet Red, and he nipped at her scratching her open hand. We were shocked as he'd never done this before, and assumed it was due to her putting her hand through the fence. We leashed him immediately and were going to move him to the car or somewhere else, when about 5 minutes after, another friend proclaimed he was 'awesome with dogs' and stuck his hand toward Red's face quickly - Red bit him, this time puncturing his finger with 2 or 3 teeth. Horrified, we put him in the car, and when we got home, we called a trainer/behaviorist and made an appointment. Prior to that incident, he (and Kitty) had barked at strangers, but never had any other aggression toward them whether it was someone visiting our home or on walks. Red would pull when we walked him and bark, but that was it. I don't know if we missed warning signs of aggression or not to be honest, but I suspect the answer to that is yes.
We met with the trainer within 2 days of the bite. Red was much more aggressive than ever had been, pulling hard, barking, etc. The trainer was excellent, sat calmly talking to us, ignoring Red and dropping treats on the floor as Red warmed up to her. Within a few minutes, he was perfectly fine with her and we worked for a couple of hours on behavior modification - all positive stuff, and I thought wow, this is going to work. We have continued this ever since, but the aggression toward strangers and sometimes other dogs has gone up and down. We started muzzling him a couple of years ago - it has been positive, we do it with positive reinforcement and he is usually fine with it, only 'complaining' and trying to get it off on rare occasions. We do that whenever someone new comes over (not often) or when people he knows and hasn't seen for a while come over. In the few years since we've done that, he usually warms up to the new person and when he's comfortable, we remove the muzzle and things have usually been fine. We have had a couple of occasions where after removing the muzzle he's grabbed on to a pant leg, but not bitten. All in all, probably 6 or 7 incidents in 4 years including the actual bites at the family gathering.
Last week, we had a new visitor, a friend of our granddaughter. We did the usual thing - muzzle, treats, let him warm up to her. He did, to the point that he was laying in her lap licking her hands with a big smile on his face. We removed the muzzle, and he seemed perfectly fine - he sat in her lap and she petted him, rubbed his belly, etc. My wife and I left the room, and a few minutes later the kids got up to go upstairs. Red bit her on the thigh - not super hard, but 2-3 light punctures and enough to bleed. We have no idea why or if there was something that triggered it, but it broke the predictable behavior we have seen in the past. The girl and her Mom are incredibly understanding and sympathetic, as they have dogs too. Her leg swelled a bit, likely a mild infection and they went to urgent care which of course (as it should) resulted in a bite report.
We are agonizing over this, but we have all but made the decision both for Red's mental health, and ours, to have him put to sleep. I made an appointment for next week, sobbing on the phone with the poor person at the Vet's office. It is absolutely destroying us. We can't look at each other without tearing up. I feel like Red knows something is wrong (I'm sure he does) though we treat and play with him like we always do. We feel like complete failures, like it's our own fault for not doing something at some point differently. We feel like we didn't give him the life he deserves. We're sad for Kitty who will grieve his loss too, and for our family who know Red for their grief. We feel incredibly guilty, and we wish we could find any alternative that would keep Red happy and alive. The appointment is next week, I'm barely sleeping, barely eating and my wife is doing the same. I feel like the most rotten person on the planet. I don't know what I'm looking for in writing this long post. It's not sympathy or absolution I don't think. I just need to talk and share my gut-wrenching anguish I guess, and I do that better in writing.