No reason why

I've been trying to be friends with my ex but we keep getting too close and make it sexual again and end up getting hurt again when I ask her why can't we just be happy together because we are always happy spending so much time together she just says she doesn't want to be with me. I ask her why though and she said that's why, to me thats not a real reason? Especially how we enjoy each others company so much she feels like now we can't even be friends