Here's why:
* Common nouns refer to general, non-specific people, places, or things. They 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: The ambassador greeted the dignitaries.
* Proper Noun: The United States Ambassador to France is a prominent figure.
In the first example, "ambassador" refers to any ambassador in general. In the second example, "United States Ambassador" is a specific title for a particular person.