Here's why:
* Common nouns refer to general, non-specific people, places, or things.
* Proper nouns refer to specific people, places, or things.
"Painting" refers to the general concept of the art form, not a specific painting.
Examples:
* Common noun: The painting is beautiful. (referring to any painting)
* Proper noun: I love The Starry Night by Vincent van Gogh. (referring to a specific painting)