Here's why:
* Common nouns refer to general 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. They are always capitalized.
Examples:
* Common noun: a dictionary
* Proper noun: The Oxford English Dictionary
In the first example, "dictionary" is a general term for a book containing words and their definitions. In the second example, "Oxford English Dictionary" is the specific name of a particular dictionary, making it a proper noun.