Because it takes discipline to turn your thoughts to other things. Hate is the easy way out - forgiveness, understanding, or eventual indifference take effort. Hate burns just as much emotional energy though, and wastes mental clock cycles you could've devoted to something useful. It's counter-productive, like taking poison and expecting it to kill someone else.
You can probably never forget but hating only festers the wound. Not recommended.
Well don't. Doesn't solve anything. Good luck.