Here's why:
* Common nouns refer to general categories of people, places, or things. They are not specific and are not capitalized unless they begin a sentence.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Examples:
* Common noun: book
* Proper noun: The Oxford English Dictionary
Since "dictionary" is a general term for a book of words and their definitions, it's a common noun.