Here's why:
* Common Noun: It refers to a general concept of a kind act or a benefit granted to someone.
* Concrete Noun: It can also be a concrete noun when it refers to a specific thing, like a "favor" you ask someone to do for you (e.g., "Could you do me a favor and pick up some milk?").
So, the type of noun "favor" is depends on the context in which it is used.