Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns refer to specific names of people, places, or things.
While we often think of "nature" as a singular concept, it is a general term for the physical world and all its phenomena. It's not a specific name.
Examples:
* Common Noun: The nature of the problem is complex.
* Proper Noun: We visited Yellowstone National Park, a beautiful example of nature.
In the example above, "nature" is used generally, while "Yellowstone National Park" is a specific place name.