Here's why:
* Abstract nouns represent concepts, ideas, qualities, or states. They don't refer to physical things you can count.
* Singular nouns refer to one thing. Since abstract nouns represent single concepts, they are considered singular.
Examples:
* Happiness (a single concept)
* Love (a single feeling)
* Freedom (a single idea)
* Truth (a single principle)
While you can have multiple instances of these concepts (e.g., many people experiencing happiness), the abstract noun itself remains singular.