Well my pup got grabbed by the neck and shook today by what looked like a staff/ mastiff cross My dog was off lead playing with a big bulldog and a Spaniel ( In a public park ) when he ran towards the dog, which was on a lead with an old guy walking it.
I've read the other replies to this and agree with much of what has been said, and yes, the guy clearly sounds like a prick who should be changing his own behaviour with regards to his dog, but I want to pick up on this as it happens ALL the time.
Dogs can be on-lead for many reasons. If a dog is off-lead and runs up to an on-lead dog,
your dog is creating the problem. Most don't see it this way as their dog is lovely and friendly, blah blah blah, but the on-lead dog - as mentioned before - may be on-lead for very specific reasons.
I'll give you a few examples: a dog has a health problem, let's say a spinal issue, and doesn't like other dogs jumping on them because it causes pain. When a dog jumps on them they resort to aggression...but only after having shown the oncoming dog, in multiple ways beforehand, which most of us miss, that it doesn't want to be bothered. That means the oncoming dog
isn't sufficiently socialised, as many believe their dogs to be, as it is only socialsed to like-minded dogs and not all. The dog doesn't need to actually make contact either, simply being a dog that is running towards the other is enough to frighten the on-lead dog and this again can cause the defensive aggression to appear - almost always because the off-lead dog isn't savvy to the on-lead dog's body language that is telling it not to get too near.
Or you can get dogs that have been poorly socialised and are kept on-lead for good reason. The owners should walk their dogs in largely dog-free places if they have no intention or means of working on their own dog's socialisation/training but many stubborn feckers - like the one you mentioned - believe they have a right to walk where they want. That just doesn't help things at all. A known reactive/aggressive on-lead dog should be muzzled, granted, but the off-lead dog running at it is still causing much of the problem.
And let's think about why some dogs are on-lead. It may well be that, like my last dog, it was perfectly well socialised until it got attacked by another dog and then it became concerned by unknown dogs approaching. You can work on this as an owner but it takes a lot of work, and all it takes is one off-lead 'friendly' dog to run at them and you're going back many steps in your training.
Ultimately dog owners should think not only of their own dog, but of other people's dogs as well. If your off-lead dog cannot be recalled when you call it, it should not be off-lead.
Dog's signal to each other through multiple subtle forms of body language, it's very quick and the vast majority of dog owners do not know how to interpret it - but they should. Better knowledge can mean the ability to see that another dog isn't comfortable, giving you the time to recall your dog, preventing it from troubling another. Unfortunately most people don't even know how to read a tail wag let alone anything else. Like most things, if we are gonna own something, we should do our homework about it beforehand.