Proper nouns should be capitalized because they refer to specific people, places, or things. Unlike common nouns, which are general and do not require capitalization, proper nouns are unique and distinct. Capitalizing proper nouns helps to identify and distinguish them from other words in a sentence. Examples of proper nouns include names of individuals (e.g., John, Mary), names of cities (e.g., New York, Paris), and names of specific organizations (e.g., Apple, Google).