Here's why:
* Common nouns refer to general categories of people, places, or things. "Cat" is a general term for any feline of the species *Felis catus*.
* Proper nouns are specific names of people, places, or things. They are always capitalized. Examples include "Whiskers" (the name of a specific cat) or "London" (the name of a specific city).
Let me know if you have any other grammar questions!