Dog owners. Or specifically the significant minority of dog owners who take their dogs to shared use areas and let their dog off the lead. Even though their dog is unpredictable/ excitable/easily spooked/aggressive. So there are kids on bikes, people on horses, runners and your dog is jumping up at them, running towards them, getting under their feet, simply because you don't want the inconvenience of having it on a lead. You're an arsehole. The dog isn't the problem, you are, you arsehole. And it doesn't matter if you say "He's not dangerous", or "He won't bite". I don't know that do I. And if your dog jumps up at me, it feels like it's dangerous and aggressive. So put it on a lead you fucking prick.
And as for those stupid extendable leads? Great fucking idea. Just let your dog run around on a lead that is so thin that other people can't notice it until its too late and have to turn into to Edwin fucking Moses to get past you. You're an arsehole too.
A significant minority of dog owners are arseholes. Those people annoy me. In case you couldn't tell.