Here's why:
* Common nouns refer to general things or people. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
Examples:
* Common Noun: I took the bus to school.
* Proper Noun: I rode the Greyhound bus.
In the first example, "bus" is a general term for a public transportation vehicle. In the second example, "Greyhound" is a specific bus company, making it a proper noun.