Any powerful emotions can make people never forget those who caused them. It was always hard to explain what's love. Even hating so much someone may mean loving him because we always think about this person or remember him/her. That's why I think hating maybe loving as well because it cause some kind of attachment.
We kind of have a voice inside us telling us that we hate a person. Even if we hated someone a long time ago sometimes we remember that person all the time and we can't get rid of such toughts. We wish of course to remove such memories once and for ever but it's impossible !
The feeling of hate maybe with us for a long time. But it could be useful as well. Because it will always remind us of the wrong people that we don't need in our lives. That may protect us in the future.
The hate may also show us how to act in front of those we hate in case we have to meet them. Sometimes we have to hold that feeling to not lose something like a work.
There are also things we hate. That may just from our personality. I think hating some tastes or colors maybe related not only to what happen to us because those things. But it's also related to our genetics. Maybe some of our relatives in the history hated that as well.
As you see even hating could be useful. And maybe it's a kind of loving. Who knows.