I get it. I am a trainer. Nothing does my heart more good than seeing dogs running and playing together, conflict free, enjoying each other's company.
However, for most dogs, there is a process that should be followed to safely and comfortably introduce dogs. An off-leash dog, approaching a leashed dog is NOT part of that process. It can do damage, both physiologically and psychologically to the leashed dog. It does not matter if the unleashed dog is supposedly "friendly" (in most cases I hear about, turns out they really aren't so friendly after all!). You NEVER know if the leashed dog can tolerate that situation...and if they cannot, it does not mean they should not be out walking (as long as their owner is responsible and can appropriately handle their dog).
Let your dog be off-leash in your fenced yard, in your house, on property where no one else is walking around with another dog. Train them, don't just hope for the best. Remember, they are domesticated animals and their instincts will kick in to guide them. Unless your dog is bomb-proof, they should not be off-leash in a public area, that is not designated for off-leash dogs, where everyone knows they are assuming risk and may encounter unfamiliar dogs.
Letting our dogs off-leash makes us feel good to see them have fun and freedom. But please consider what could happen if your dog runs up to a dog...the owner of the leashed dog should protect and advocate for their dog and do what it takes to spatially or physically push your dog away...yell, kick, spray them, whatever it takes. No, I don't advocate for kicking dogs and harming them, but you bet I will protect myself and my dogs, by any means possible, if an approaching dog threatens to harm us. That is on you as the owner, for willfully allowing your dog to run loose. You moment you choose to unleash your dog, you become the party morally responsible for any injuries that occur to your dog and others around you. Please think twice before allowing your dog to be unleashed and think of the potential consequences before you remove your leash. You owe it to your dog to think with your head and not your heart!