Here's why:
* Common nouns are general names for people, places, things, or ideas.
* Proper nouns are specific names for individual people, places, things, or ideas and are capitalized.
"Ambition" is a general term for a strong desire to achieve something. It's not a specific name, so it's a common noun.
Examples:
* Common noun: Her ambition is to become a doctor.
* Proper noun: Dr. Smith is a well-known surgeon.