Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are part of a proper noun phrase.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
Examples:
* Common noun: instructor, city, dog
* Proper noun: Professor Smith, New York City, Fido
Since "instructor" refers to any person who teaches, it is a common noun.