Our big guy used to go bonkers, whirling and snapping on his leash and he corrupted our other dog into reacting similarly. The last straw was when they pulled me over and dragged me 30 feet on my belly over the snow to get to a little fluffball that was having an equivalent fuss. I took the non-grey to basic obedience where they taught us to distract with a treat when we encountered another dog. I had to just give in and travel with a cookie bag, and over time they learned to look at me automatically when they saw or heard a barking dog on walks. Worked like a damn. I still carry the cookie bag just in case, but it did eradicate the behaviour. Takes time but it works!