Here's why:
* Common nouns refer to general categories of people, places, things, or ideas. They are not capitalized unless they are part of a proper name.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
For example:
* Preschool is a general term for a type of educational institution.
* Sunshine Preschool is a specific name of a preschool, therefore it's a proper noun.
Let me know if you have any other questions!