LEASH ONLY DOG AGRESSION

Question
MY ALMOST 2 YEAR OLD GOLDEN RETRIEVER IS SWEET AND OBEDIENT. HE IS NEUTERED AND LOVES TO PLAY WITH OTHER DOGS IN THE PARK, AS LONG AS HE IS OFF HIS LEASH. WHEN HE IS ON HIS LEASH HE IS SPORATICALLY HORRIBLY AGGRESSIVE. SOMETIMES I AM SITTING PEACEFULLY WITH HIM ON HIS LEASH AND HE LETS MANY OTHER DOGS GO BY AND DOESNT UTTER A GROWL, YET ANOTHER DOG OR PUPPY WILL WALK BY AND SUDDENLY HE LUNGES GROWLS AND ATTACKS A PERFECTLY INNOCENT DOG. TONIGHT I GOT TOO CLOSE AND MY LEG GOT SCRATCHED REALLY BAD. I DO NOT KNOW WHAT TO DO.

Answer
Your dog is showing you that you are his Alpha dog and he wants only to protect you.  Try clicker training, Goldies do great with it. Get a clicker (cheap ones at some pet stores and online) and some small training treats in your pocket or pouch. When he allows another dog to walk near you peacefully on leash, click, "Good boy ___dog's name__ Stay", and give him a treat. If he moves towards the other dog, give a firm "NO", then ask him to sit back down next to you.  He is trying to serve you well by protecting you.  It is up to you to teach him what is appropriate.