This is a very common question. The first thing to know is that this behaviour most often is based from a lack of confidence or anxiety; being on the lead can compound these feelings as the option to run away isn’t there.
Dogs have their own body language that can be easy to miss. Licking their lips, yawning, turning their head away are all some of the ways they could be saying “Hey, I’m feeling a little nervous but don’t worry I’m friendly”. Imagine the situation, you’re walking along a path and meet another dog coming towards you. Your dog starts showing its calming signals yet still the other dog is coming straight at them! Eventually they feel so threatened that they feel all they can do is to bark to make them go away. It’s perhaps easy to see how this becomes default behaviour, as it seems to be the only thing that actually works.
How can you help them?
Firstly, understanding the motivations for this behaviour should make a huge difference. Your dog could wear a yellow lead – which is becoming known as a signal to other dog owners to respect that your dog needs more space when on the lead. Try to avoid areas where conflict is more likely or change direction if you think they will react badly. Encourage your dog to be more focused on you (with treats) rather than the other dog.
Through carefully supervised training, you can work to help your dog overcome their negative emotions and teach them new behaviours to use instead. And slowly, you can re-educate your dog to be more relaxed to the sight of other dogs. Whilst it’s unfair to say you can ‘cure’ it, you can though help your dog to be more confident and able to make better choices.