Here's why:
* Common nouns refer to general things or people, not specific ones.
* "Bridge" refers to a structure built to span a physical obstacle (like a river, road, or valley).
Examples:
* The bridge was built in the 19th century. (general reference to a bridge)
* We crossed the bridge to get to the other side. (general reference to a bridge)
Compare this to a proper noun:
* The Golden Gate Bridge is a famous landmark. (specific bridge)
Let me know if you have any other questions!