Here's why:
* Common nouns refer to general, everyday things, places, or ideas. They are not capitalized unless they are the first word of a sentence.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
For example:
* Flood is a general term for a large amount of water covering an area.
* The Great Flood would be a proper noun because it refers to a specific historical event.