Here's why:
* Common nouns refer to general, non-specific things or people. They are not capitalized unless they are the first word of a sentence.
* Proper nouns refer to specific people, places, or things, and they are capitalized.
Examples:
* Common noun: I bought a new canteen for hiking.
* Proper noun: I went to the Canteen restaurant for lunch.
In the first example, "canteen" refers to any type of water container. In the second example, "Canteen" refers to a specific restaurant.