leash aggression towards humans

Question
My 2 year old neutered male cocker spaniel has in the past few months developed what I can only refer to as leash aggression.  He is normally a loving dog and loves to be petted, played with and cuddled.  However, when I am out walking him on the leash he is extremely uncomfortable with people approaching and petting him. Even if he has seen these people numerous times outside and I have stopped and talked with them.  If they have been inside the apartment he is fine with them and loves them all the same. However, if our only contact has been outside...he is extremely uncomfortable.  An example is my neighbor was out and she walked up to talk to us.  She has seen Sammy several times and Sammy has seen her.  She bent down to pet him and he avoided her,tucked his tail under, growled and then snapped at her.  PLEASE HELP!

Answer
Early socialization is very important, but dog's personalities are not fully developed until 3 years old.  At his age, he is taking on more responsibility to protect you and him.  Don't let people force their attention on him.  If he withdraws, let him.  People don't have an unlimited right to pet any dog they encounter.  

Having a good pack structure reduces such problems.  The dogs see all the
people and dogs in the household as a pack with each having their own rank in
the pack and a top dog.  Life is much easier if the 2 legged pack members
outrank the 4 legged ones.  You can learn to play the role of top dog by
reading some books or going to a good obedience class. A good obedience class
or book is about you being top dog, not about rewarding standard commands with
a treat. Start at http://www.dogsbestfriend.com/  If you are showing strong leadership, then he needs to do less about people he sees as problems.  You need to greet people as positively as you can.  Relax and don't let your self tense up.  You can't hide your emotions from your dog, you must control them.  With your neighbor and others you know, you might have them give him treats.  Nothing elaborate, unless spoiled even a few of his regular kibbles may be enough.  I have heard of using Cheerios.