Here's why:
* Common nouns refer to general things, not specific individuals or places. Examples:
* dog (not a specific dog, just any dog)
* city (not a specific city, just any city)
* book (not a specific book, just any book)
* Proper nouns, on the other hand, refer to specific people, places, or things. They are always capitalized. Examples:
* Fido (a specific dog)
* New York City (a specific city)
* Harry Potter and the Sorcerer's Stone (a specific book)
Let me know if you have any other questions!