Here's why:
* Pronouns stand in for nouns.
* Possessive pronouns show ownership or belonging.
Examples:
* "This book is yours." (Yours replaces the noun "your book")
* "The car is mine." (Mine replaces the noun "my car")
While "yours" is technically in the possessive case, it doesn't have a separate nominative or objective case like some other pronouns (like "I" vs. "me"). It always functions as a possessive pronoun.