Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are the first word in a sentence or part of a proper noun.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
For example:
* Common noun: dog
* Proper noun: Fido (a specific dog)
* Common noun: cat
* Proper noun: Mittens (a specific cat)