Here's why:
* Common nouns refer to general, non-specific people, places, or things.
* Proper nouns are specific names of people, places, or things and are always capitalized.
Examples:
* Common noun: author
* Proper noun: J.K. Rowling (a specific author)
So, "author" refers to anyone who writes a book, while "J.K. Rowling" is the specific name of a particular author.