Hello, I have a 4-year-old bullmastiff who I bought 3 months ago. He has started barking at people who walk by in the house and the car and barks at people when we're out on walks. I believe he's trying to protect me as I am a somewhat timid person. I want to stop this behavior. I took him to obedience classes which helped a lot for general obedience but he still barks. What do I do?