Mine does it too. He chases cars (louder and bigger the better), motocycles, bicycles, skateboards, and even people running. My biggest is the other dogs. When he sees other dogs, he goes crazy. I don't think he is being aggressive. It is more like he wants to be close to other dogs. But him barking, crying and pulling makes other owners feel uncomfortable for us to come near. The only time he completely ignored passing dogs was when I had a Korean BBQ rib bones. He was so into licking it and trying to get it out of my hand, he didn't even bother to look at the dog who was passing right next to him. With other treats, he ignores them and lunges at the other dogs. I can't carry Korean BBQ rib bones all the time (I may attract entire neighborhood dogs). So far I haven't found any other food that made him stop focusing on other dogs.