Here's why:
* Objective Case: Refers to the object of a verb or preposition (e.g., "I saw *him*.").
* Nominative Case: Refers to the subject of a verb (e.g., "*She* went to the store.").
* Possessive Case: Shows ownership or belonging (e.g., "That's *my* car.").
"Yours" indicates ownership or belonging, as in "This book is yours."