Here's why:
* Proper nouns are names of specific people, places, or things. They are always capitalized.
* "Rover" is a common name for dogs, but when it is used as the name of a specific dog, it becomes a proper noun.
Example:
* "My dog's name is Rover." (Rover is a proper noun)
However, "rover" can also be a common noun when it is used to refer to any dog, or to someone who roams or wanders.
Example:
* "The rover barked at the mailman." (Rover is a common noun)
* "She was a real rover, always traveling to new places." (Rover is a common noun)