Ok, I think I need to work out what's going on here and how to sort it. Jasper is never aggressive on lead. But when off lead, and chasing his ball, he can be very grumpy to other dogs that try to join in. So, other dog approaches to have a sniff or whatever, and he ignores them and moves away. But if the dog persists, or starts chasing the ball, he will turn around and jump at their face with a lot of noise and mouthing. He hasn't bitten or injured another dog, but it's a very clear 'Fuck off'. He's now a year old, and seems less tolerant of puppies than older dogs. But yesterday he took on a rottweiler and I had visions of having to rescue him from down its throat. Luckily, the rottie was a friendly girl who snorted at him in disgust and pissed on his ball in retaliation. Today, a collie puppy approached, started barking at him (I assume he wanted to play?) and Jasper lunged at his neck and the pup submitted. I was mortified and apologised profusely. The owner was fine about it, but I'm beginning to think this is a growing problem and I need to do something. I've googled, and there's a lot of pack stuff about leadership etc that I don't really understand. Any advice? He is ball obsessed, and I'm reluctant to give up the ball because he gets so much exercise and joy out of it.