Loud neighbors can keep people up throughout the night. With sleep deprivation causing some people to be irritable, snappy, less patient, the effects spread to everyone the affected person interacts with. This causes all around bad mood, making people's days miserable. Another way loud neighbors hurt is also indirectly. Lack of sleep can also make people slower and less efficient. With that in mind, if the continuous loudness continues, those effects could cost the affected person their job due to inefficiency.
