Dog Behaves Badly on Walk

Question
When we pass another dog during our walk, my dog will lunge, bark and growl
at the other dog.  This is embarrasing and hard to control since i have a large
golden retrieve mix.  I don't  understand the behavior, at the dog park my dog is
the friendliest one there!  Usually when I see another dog coming I make him
sit/stay but as soon as the other dog comes closer the behavior begins.  Any
suggestions

Answer
Im sorry I can't really do anything over the internet. I have to be there to observe the bahavior. One thing I could suggest is what I would do. When this happens, observe what happens, when it happens and when it stops. Try to find what the cause is before you try to stop the behavior. After that, you need to work that information. If you need help, just email me.