Why do dogs who wouldn't hurt anyone bark like crazy at other dogs on-leash?

We’ve all seen or had some version of this dog. They act like they’re going to shred another dog to pieces but only when they’re on-leash. If this is your dog, you might even know that if you approached the other dog everything would be fine, but there’s no way you could politely let your dog approach someone while they’re acting that way. Why does this happen only on leash for so many dogs?