When we took him to the park 2 days ago, we let him off the leash with my cousins dog and every time the other dog got too close, my dog started barking and tryin to bite it. My cousins dog is only a puppy. And while we were walking him back, another dog ran up to him. My dog sniffed him and waged his tail and then started barking. But when he is behind a dog he tries to sniff there butt. What’s wrong with my dog? How do I stop him behaving like this?