Here's why:
* Common nouns refer to general, non-specific things. They are not capitalized unless they are the first word of a sentence.
* Proper nouns refer to specific people, places, or things, and are always capitalized.
Examples:
* Common noun: The sail was torn in the storm.
* Proper noun: The Titanic had three huge sails.
In the first sentence, "sail" refers to any sail in general. In the second sentence, "Titanic" is a proper noun, but "sails" is still a common noun.