Here's why:
* Common nouns refer to general categories of people, places, things, or ideas. They are not capitalized unless they start a sentence.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
For example:
* Violin is a general category of musical instrument.
* Stradivarius is a specific brand of violin (a proper noun).
Let me know if you have any other questions!