We had this with Honey when she hit adolescence, but I'm not sure I can really advise as I'm not entirely sure how we managed to overcome it...

What we did was stop taking treats out with us for a while (made recall training and loose lead walking really difficult) as this made us less of a special object. We also kept our distance from other dogs for a while... if they approached us we didn't fuss them or speak to them at all. We praised Honey for all positive interactions with dogs - at first even just being near a dog without reacting and gradually positioning us closer to dogs...
We walked in tandem with some willing dog owners and their dogs, so that her warning the other dog off didn't succeed in getting rid of it... And whenever it was safe to do so, any growl or snap to another dog meant that we walked away from Honey, so she didn't gain our undivided attention from it.
At the same time we ensured we were generally more consistent with her (some areas had slipped to be honest

) and applied the Nothing In Life Is Free philosophy, so she didn't get fuss/play/food unless she earned it.
Once we got through the guarding we found we had a dog who had lost all her confidence, so I don't know if the guarding behaviour was a mask for that, and we are still working on improving her reactions in some situations, but we can now greet and fuss dogs we meet, as long as we don't make too big a deal of it... (and they aren't rude, i.e. jumping up!).
Honey was DEFINATELY worse with puppies (I think unwittingly we are more enthusiastic about meeting puppies and it makes the situation more stressful for the other dog)... and she will still guard us from other dogs in her own home (sorry, Jarvis!

) - in this situation I would definately suggest that all dogs lose your attention if there is any quarreling.... just remove yourself from all of them at the first sign of growling... he'll quickly learn it's counterproductive... (likewise praise tolerant behaviour, so he knows that's what you want him to do!).
So much of dog training relies on creating a strong bond between dog and owner, so I guess it's to be expected that some dogs find it hard to share the most important person in their lives...
I don't know what did actually work, and can't guarantee that any of it will work for anyone else.
All I can add is that, if we had not seen a major improvement in this behaviour in the 2 weeks it took to get it under control, we would have been straight to a behaviourist, as it is, unfortunately, a very self-rewarding behaviour and could easily have escalated... I had the contact details and price list for a local behaviourist all ready to make the call!!

Editted to add - if you do need to go down the behaviourist route, make sure you find a suitably qualified one who uses positive methods - this kind of behaviour is one of the prime targets of rattle bottle/water pistol modification, which could end up escalating the behaviour into a serious dog-agression issue!
Jean Donaldson's book "Fight" covers resource guarding between dogs (including human's as the resource) if you can get hold of a copy...