Here's why:
* Common nouns refer to general things (e.g., dog, city, happiness).
* Proper nouns refer to specific people, places, or things (e.g., Fido, London, Christmas).
"Dear" describes something as beloved or cherished. It's often used in greetings, but it's not a noun itself.
For example, in the phrase "Dear John," "Dear" modifies the name "John" and expresses affection.