Here's why:
* Common nouns refer to general people, places, or things. They are not capitalized unless they start a sentence.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
Examples:
* Common noun: The student was late for class.
* Proper noun: John is a student at Harvard University.
In the first example, "student" is a general term for anyone who is learning. In the second example, "John," "Harvard University" are specific names and therefore proper nouns.