Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized.
* Examples: John, London, Amazon River, The Great Wall of China.
* Common nouns are general names for people, places, or things. They are not capitalized unless they are at the beginning of a sentence or part of a title.
* Examples: boy, city, river, wall.
"Bookshelf" is a common noun because it's a general term for a piece of furniture that holds books. It's not a specific name.