Here's why:
* Possessive adjectives in French indicate ownership. They change to match the gender and number of the noun they modify.
* The noun being described is what determines the form of the adjective, not the person who owns it.
Example:
* My book:
* Masculine singular: *Mon livre*
* Feminine singular: *Ma robe*
* Masculine plural: *Mes livres*
* Feminine plural: *Mes robes*
In this example, the possessive adjective "mon/ma/mes" changes to agree with the gender and number of the nouns "livre" (book) and "robe" (dress).
Note: It's important to remember that the possessive adjectives also change depending on the person they refer to (I, you, he/she, we, you, they). However, the agreement with gender and number always applies to the noun being described.