Why is it sometimes hard to let go of past love?

It's simply natural for us as humans to fall in love, and sometimes, when we fall for someone, we fall hard. There's absolutely nothing wrong with that, though it can make the healing process uncomfortable and taxing. However, know that your heart -will- heal with time; just continue having hoping, and looking forward for a brighter tomorrow. Who knows when you'll fall in love again, and maybe this time they'll be the one!

Depence how long you with that person how deep your feeling and afecvtion was of if that person meant the world to you and died those are mime hard to go issues

Because at one time in your life it's what made you extremely happy, you want to relieve that feeling again and those memories. It still hurts I suppose - even if the situation itself ended on good terms, it hurts knowing that you once had something special and rare, but that you can't go back and change it, all you can do is keep the memories and change your 'today'.