Do You Have a Dog Who Reacts Badly to Other Dogs or People? Dog reactivity is quite common, but what causes it? Here's advice from Canine Behaviourist Sydney. There are many different reasons for reactivity towards other dogs or people. The most common ones are lack of socialisation when young, bad experiences and frustration due over-excitement. These are the most common issues I notice in dogs that I train. Whether the reactivity can be completely fixed depends on how long it's been going on and how severe it is.