Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things, and they are always capitalized.
"Sandwich" is a general term for a food item made with two pieces of bread with a filling.
Examples of proper nouns related to sandwiches:
* Subway (a specific sandwich shop chain)
* The Reuben (a specific type of sandwich)
* Aunt Jemima's (a specific brand of bread)