Because there are so many more people that deserve hate.
It's harder to deal with being hurt if you love someone and the loved one hurts you.
When you say 'I hate you', you say it almost subconsciously, only feeling the hurt they've caused you.
When you say 'I love you', you have to pull down you barriers and put all your emotions and feelings into those three words.
People put walls up for a reason. The longer they are up, the harder they are to take down.
Its easier to hate because we don't have to feel.
Its just another person we are removing from our lives.