How can I make my dog stop barking at friendly strangers that want to pet him when we are on walks?

My dog was not always a barker. He is now very tuned in to my emotions. If I gasp or laugh loudly while watching TV, he runs out of his doggy door and barks at nothing. Also, he has started barking at strangers who wish to pet him. This is obviously the bigger problem. He barks and will not let them pet him. I am not sure why this started because I can't think of anything that has changed.