One of the most common ways that dogs communicate to one another is through barking. The most common reason why dogs bark is because they are protecting their territory, whether it be property or family. If a dog feels threatened, this may trigger excessive barking. As the threat becomes closer, the barking will often get louder and more aggressive.
Dogs are pack animals, meaning they thrive on companionship and do not like being alone. If a dog is left alone for a long period of time, they become lonely or bored and may bark. This kind of barking is done in hopes that someone will hear them and come for them.