Some dogs just don't feel that comfortable around other dogs. Sometimes they've never spent time around other dogs so don't understand the subtle give and take that goes on between dogs, or maybe they just aren't great with reading other dogs to begin with, which many introverts can identify with. But the bottom line is that when a dog has anxiety around other dogs, it's the human's job to intervene.
O'Keefe is one of those dogs who I don't think has ever spent much time around other dogs. He is noticeably anxious when he's around them and with him the stress causes him to get bouncy and to lose track of where his body is in space. Eventually his clumsiness causes him to run into or land on another dog and because he's already stressy, his reaction is to snap at them. In a situation like that his stress has set him up to reinforce his belief that other dogs can't be trusted.
As any of you know, when you practice a habit regularly, that habit becomes more and more difficult to break because the longer you do it, the more frequently you are being reinforced for it. In O'Keefe's case I wouldn't call him a reactive dog because he really does his best to ignore the situation and move on. He does great on leash and can pass other dogs on walks without reacting toward the dog - he's nervous but he passes them by quietly. However, O'Keefe is definitely a dog who is teetering on the edge of becoming reactive if he continues to have negative experiences with other dogs.
I felt really bad the other day because I took O'Keefe to a meet up and he was anxious about it. It was a new place and he was meeting new people so I should have know better than to say yes when another person who happened to be there as well and asked if her two dogs could meet him. I'll give him this, he did admirably well. He stepped back a few times, was looking away (a way a dog signals that they mean no harm and want to diffuse a situation), but the other large dog, a lovely huge, elderly Golden Retriever, kept bumbling forward to come meet him. He felt surrounded and then finally he snapped in his face. My fault. I set poor O'Keefe up and it wasn't fair.
Does that mean O'Keefe can never be around other dogs? No way. He's currently living with four of them. But what it does mean is that it is going to be of the utmost importance that from now on, as much as is humanly possible his meetups with other dogs will need to be short, carefully supervised, not face to face (dog's should never greet each other face to face like they typically do on leash) and then rewarded with a high value treat after a few seconds of greeting as he is moved along. That will build up a connection for him that other dogs foretell good things. This will be reinforcing a new mindset for him since currently his conditioned emotional response to dogs is to get nervous. If he learns that dogs = yummy things, much like Pavlov taught his dog that a bell = food and built a response, he will soon feel a positive emotion when he sees another dog and will start to look to his human for that reinforcement.
Will he always need the treat when he sees a dog? Nope. When that response is strong and happening every time then the value of the reinforcement can be varied as well as how often the reinforcement happens. This will really strengthen his behavioral response.
Consistency is the key though. Animals, us included, are never static. A behavior is always becoming more likely or less likely - the trick is to move that behavior in the direction we want it to move. And we can - all with positive reinforcement. The toughest part of R+ training though is for the human to change their own personal habits and make sure they are consistent. I know I mess up occasionally, like I did at the meet up but changing that and doing what is best for the dog is key so I've learned my lesson and am being much more careful. The result is a more relaxed O'Keefe (snoozing beside me currently as we type) who is learning being around other dogs doesn't have to be so worrisome.