Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things, and are always capitalized.
Since "German Shepherd" refers to a breed of dog, it's a general category, making it a common noun.
Example:
* Common noun: We have a German Shepherd.
* Proper noun: My dog's name is Max, and he's a German Shepherd.