"It is easier to forgive an enemy than to forgive a friend."
  William Blake

The only thing that I can think of as an explanation is the fact that you can easily forget that enemy, but you will never forget that friend, making it harder to forgive. 

Why is it so hard to let go of ill feelings against someone? I still harbor feelings against a woman who didn't like me twenty five years ago. I doubt that she has thought of me in years!