Here's why:
* Common nouns refer to general things or people. They are not specific to a particular individual or place.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Since "face" refers to the general feature found on all humans (and some animals), it's a common noun.
Examples:
* Common noun: The man had a kind face.
* Proper noun: John had a kind face.