WHY DOES MY DOG BARK AT STRANGERS WHEN OUT ON WALKS?

Your dog is protecting you and himself by warning off what he perceives as potential aggressors. It's natural behaviour, but unacceptable in a human environment. Your dog has not been socialized adequately with other people, so he treats them all as "foes" when out of the safety of his home environment.

The best way of overcoming this overprotective behaviour is to take him to socialization/training classes on a regular basis, where he'll meet other people and dogs in a controlled environment and learn to relax in their company