Most experts on dogs will tell you that separation anxiety is the most important cause of excessive barking. If you spend a lot of time with him on the weekends and then work all week and just tend to his needs such as food and water he will continue to feel anxious. Your dog may just feel neglected. Puppies and even adult dogs grow very attached to their families and require a lot of love and attention. Some dogs will bark to protect their territory such as their yard and bark when they think someone strange is entering their territory.
Dogs can be very protective of their homes.
Once a dog has been successful in scaring away an intruder, then he is more motivated to bark again when another person walks by his territory. He is just trying to protect his own turf. Some breeds of dogs are prone to barking more than others, usually Dobermans and Rottweilers bark very little and Beagles and Terriors are big barkers. Especially young dogs like to bark also as they are usually very alert and responsive to noises and other people or animals. The larger and slower dogs bark less than the smaller ones and the fast ones.