lab aggression

Question
We have a 10 month yellow lab.  We love this dog and he has become part of our family.  As a puppy he showed no agression, now as he has gotton older we have a problem.  He loves other dogs our problem is with people.  People will automatically think all labs are nice.  They will come over to pet him.  He looks nice, sometimes he will let you pet him, then out of the blue his hair goes up and he tries to bite them.  He is very unpredictable.  This is very embarassing and upsetting because we love to take him on walks and go camping.  Please help us to retrain him.  He is easily trained:he knows how to sit, stay, roll over and sneak.

Answer
It's hard to comment without being able to see this type of behavior in person.  

The best I can offer is enroll him in an obedience class. Don't try retraining yourself.  And when you make the call about what classes to enroll in, talk to the school/instructor and let them know that the dog has been showing aggression issues towards people.

I would also consider calling the breeder, if possible.  They need to know and they may also be able to offer some advice.  

You need outside help who can work with you and your dog in person.  Close attention needs to be paid to see if the dog is "guarding" you as a resource or perhaps he is feeling threatened by people who approach.  When he gets aggressive "out of the blue", did the folks sneak up on him?  Maybe he has a problem seeing?  If the behavior is new and only with strangers, perhaps something happened when he was out of your care, like if you board him.  You may also want the vet to do an overall health exam, because sometimes sudden changes of temperament can indicate the dog isn't feeling well.