Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized.
* Common nouns are general names for people, places, or things. They are not capitalized.
Example:
* John (proper noun) is a person's name.
* Man (common noun) is a general term for any male human.
So, "John" is a proper noun because it is the specific name of a person.