So talk to a sheep farmer, they'll tell you the presence of an unfamiliar dog when their chasing instincts kick in, and especially during lambing season is a source of stress for the animals, sufficient in some cases to lead to the loss of the lamb. Why is it some consider it acceptable that people should be entitled to enter their land without having a little respect? It's not like they'd be too happy if strangers turned up on their doorstep allowing their dogs to do as they please on their property.
The owners of the majority of dogs who worry sheep or attack people would have considered them well trained and obedient until it's too late.