Lurcherboy is have somewhat of a recall regression since we lost oldgirl. He's missing having a canine playmate and wants to say hello to every dog we meet - but, I don't allow him to.
If another dog is off lead near us and the owner doesn't put him/her on the lead when they see us approaching, there is usually some sort of eye contact, head-nod and smile type signal between owners that confirms it's ok to let him play. If I read any other body language from dog or owner I don't let lurcherboy approach.
It's actually quite funny at the moment, as having changed dog-walking times I keep coming across the same few dog-walkers, but we always seem to be equally spaced out around the field and just that bit too far for both owners to be near any playing, iyswim. So, we are all keeping our dogs loose, but close, despite the fact that all indications are that the dogs will probably place nicely if they ever get close enough to meet. Once we have met them and allowed the dogs to play, I will still check with the owner if it's ok for my dog to play with theirs, as you never know when a dog may be having an off day, might have had a bad experience with another dog or is ill or injured etc.
It's not easy with him being canine pal deprived just at the moment, as his motivation to play with other dogs is so high, that it is almost trumping his usual rewards. So, I have to watch his body language carefully and if I see him even think about approaching an on-lead dog, or anything else inappropriate, I have to put him on the lead for a few seconds until we've passed whatever is catching his attention.
I think it's important to remember that dogs cannot behave or communicate naturally when restricted by a lead and therefore many otherwise perfectly friendly dogs can react badly to being approached on-lead by an over enthusiastic dog. The on-lead dog is unable to give off clear enough body language and/or escape the other dog's advances, so they resort to the only option they have. If you just ask the other dog owner if their dog would like to play, then your dog can enjoy meeting and playing with theirs without anyone needing to worry.
I think someone upthread asked how you get your dog to recall when they want to play with another dog. The answer is through lots of hard work and training, plus finding some way to make yourself as interesting as you can - hopefully more interesting than their potential playmate - not an easy task. In our case we have squeaky toys that are their top-grade rewards and they are only allowed to play with on very select occasions. One squeak of their respective toys and they are back like a shot for the rare chance to play with their beloved toy. It's a case of finding out what most motivates your dog in particular, for some it may be food, for others toys/games. It's also important to remember not to try free-running recall in the park etc until your pup has gone through the stages of developing a bombproof recall in the home and garden, where there are less distractions. Then you need to up the ante to more distractions, but in a secure, controllable environment, before moving on to off-lead in public parks etc.
I would also highly recommend training the instant down, as not only can it save your dog's life if, for example, they are running towards traffic, but it is also an invaluable way of stopping them getting themselves into all manner of hot-water, including approaching on lead dogs. Lurcherboy actually responds better to the instant down than the recall at the moment, so I am able to use that to stop him approaching leashed dogs if necessary.