Here's why:
* Abstract nouns refer to concepts, qualities, feelings, or ideas that cannot be touched or physically seen. Kindness is a quality or virtue, not a physical thing.
* Common nouns are general names for people, places, or things (e.g., dog, city, book).
* Proper nouns are specific names for people, places, or things (e.g., Fido, Paris, Harry Potter).
* Collective nouns refer to a group of people or things (e.g., team, flock, committee).
Since "kindness" doesn't fit any of the other categories, it's an abstract noun.