Dogs bark at people for several main reasons including territorial behavior, fear or anxiety, excitement, protective instincts, attention-seeking, and lack of socialization.

• Dogs naturally perceive unfamiliar people entering what they consider their territory as a potential threat, so they bark to warn intruders and alert their owners.

• Some dogs bark out of fear or anxiety if they are unsure or frightened by strangers, especially if they have had negative past experiences or lack proper socialization.

• Barking can also express excitement and a desire for social interaction, when dogs are happy or eager to meet new people.

• Protective or guarding instincts also trigger barking in certain breeds that are naturally watchful and alert for their family.

• Dogs may bark to get attention from their owners or strangers, having learned that barking results in being noticed or rewarded.

• Additionally, dogs may bark at certain people more than others due to a person's appearance, behavior, or scent evoking a specific past memory or feeling in the dog.

Overall, barking is a complex, natural form of canine communication used to express a range of emotions and intentions toward people. Understanding the root cause helps in managing or training the dog to bark less at people when needed. This summary is based on multiple recent expert sources about dog behavior and training.