I was confused about this too, & saw a great explanation in Colloquial Hebrew book. Paraphrasing, If you need the direct object et & a personal pronoun, you combine them together and get oti etc. So if it were otherwise: I saw her. Since her is a specific person you'd need a direct object "et" before her, they get combined, the Hebrew version of a contraction... .אני רואה אותה . We love him
אנחנו אוהבים אותו.