If your dog keeps on getting attacked, or picked on, by other dogs then it is probably suffering from low confidence.
Most dogs attack weakness and respect strength (confidence) hence the reason why they respond well to calm assertive leadership. Now this is not dominance per se as they are not looking to assert rank but, in keeping with their survival principles, looking to ensure the pack does not appear weak.
Years gone by weaker dogs would have had two choices if they wanted to survive. They could either become stronger and more confident and thus add value to the pack or leave it altogether.
Nowadays dogs don’t run around in packs but a significant amount of dogs have never lost this instinct and will instinctively look to repel any, and all, weakness that comes into contact with them. This creates a ‘victim complex’ in low-confidence dogs and will only get worse if not addressed.
If your dog is one of these dogs that always seem to be the target of rough behaviour from other dogs then it need not be the end of the world. There are two ways you can deal with this. The first, which many guardians adopt but is not one I would recommend for long term harmony, is to totally avoid other dogs and only go out when you know the likelihood of coming across other dogs is low. Adopting this approach will reduce the risk of your dog being attacked by the sheer fact it will not come across any other dogs. However, you are doing nothing to address the cause and provide a remedy to it.
The option I would prefer is to build your dog’s confidence and, by doing so, it will not be seen as a target by other dogs going forward. This does not mean that you need to “toughen up” your dog and for it to appear physically “stronger” than other dogs as this will not help as it will no doubt increase the aggression in your dog and thus this risk of your dog causing conflict.
No the best way to increase your dog’s confidence is by allowing your dog to make it’s own choices and rewarding the choices it makes that meet with your approval. For example, teaching your dog to look at you on command and rewarding it for doing so. This simple ‘game’ will not only increase the bond between you and your dog and increase your dog’s confidence, as you are rewarding its (correct) decision making, but it is a tactic you can deploy if you see something in the distance that you know will upset your dog. If you get your dog to focus on you it will not be looking at the other dog/person/cat etc and you will be able to reward it for doing so and retain control of the whole situation.
I go into more detail on this in my ‘eyes on you‘ blog.
The more games you play, the more choices your dog has to make and the more you reward the ‘correct’ choices the more confident your dog will become and this WILL be noticed by other dogs and lessen the risk of your dog becoming the ‘weakness’ target. To ensure that you have the best chance of embedding this confidence then make sure you use the 5 Cs of optimum dog communication when interacting with your dog. A strong leader makes for strong followers.