Here's why:
* Common nouns refer to general things or ideas, not specific ones. They are not capitalized unless they start a sentence.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Examples:
* Common noun: quiver (a case for holding arrows)
* Proper noun: Shakespeare (a specific playwright)
Since "quivers" refers to any case for holding arrows, it's a common noun.