Here's why:
* Common nouns refer to general, non-specific things. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns refer to specific people, places, or things and are always capitalized.
Examples:
* Common Noun: lake (refers to any lake in general)
* Proper Noun: Lake Superior (refers to a specific lake)
Let me know if you have any other grammar questions!