Comment by bradrn

> Can anyone explain why this happens?

Historically speaking, it seems to have been due to two independent coincidences:

• The falling together of object pronoun her and possessive her seems to date back to Old English, where the genitive and dative cases were both hire. I’m unsure how this originated, but they seem to be separate in Frisian (the closest relative of English) and were reconstructed as separate in Proto-Germanic, which suggests that it was due to regular sound change in the evolution of Old English.

• Later, it seems like the third person possessive pronouns were derived by adding ’s to the corresponding genitive forms (which ended up as the possessive determiners). So her became hers, but it had no effect on his, since that already ended with an ⟨s⟩.

Thus we end up with the paradigm of:

    |   | subject | object | poss pronoun | poss determiner |
    |---+---------+--------+--------------+-----------------|
    | M | he      | him    |             his                |
    | F | she     |       her             | hers            |