Here's why:
* Common nouns refer to general, everyday things or people. They are not specific names.
* Proper nouns are specific names of people, places, or things (e.g., Dr. Smith, New York City, The Eiffel Tower).
Since "dentist" refers to any person who practices dentistry, it is a common noun.
Abstract nouns refer to ideas, qualities, or concepts (e.g., happiness, love, justice). "Dentist" doesn't fit this category.