Carlos Castaneda's character, Don Juan, taught the narrator about teachers.  The things I learned reading it have percolated for decades since.  
He speaks of gratitude to our most mettlesome enemies because they are the teachers who give us the most value.  A popular song by Christina Aguliera has her singing her thanks to the one who broke her heart for making her "faster, stronger, thicker skinned and a fighter."  This is the essence of Don Juan's teaching on teachers.
We know who our mentors are and we honor them both deliberately and indirectly through the quality of our characters.  We don't honor those who teach us by adversity, however.  They seem to do it out of the meanness of their hearts, yet what price do they pay to offer you the teaching you recieve?  Have you yet failed to learn an important lesson at the hands of those least friendly in your experience?
Not all of those who cause us grief, and thereby teach, are our enemies.  Some are simply the hapless agents of your destiny, no more happy to play the role than you are to recieve the learning.
The greatest value we could take from learning gratitude for the lessons offered in our worst wounds is the calmness it would afford us ourselves.  
When we hate it is an emotional agent for one's desire to cause the removal of those we hate.  We think of them as being detrimental to our world when yet they are serving a valuable, if odious, purpose in the lives of those around them.  The interactions they have make deep impact and teach deeply into the soul.  
Not all lessons are welcome nor immediately apparent yet inevitably, if we survive our education long enough, we are able to see how we have become stronger, more sure, more focussed and more enduring because of the unpleasant experiences we have had.
It is feeling that one has, without any questions, the ability to weather any possible storm, that brings us our greatest ease and power.  To know that no pain is more than we can bear, or that no load is too heavy to carry, this gives us more power and security than an army of men and heavy fortifications could ever do.  
We can step out into the world with courage and anticipation instead of hiding behind our walls where nothing hurts us dreading every thistle's sharp bite in the world outside.
Those who have been our worst enemies have also been our greatest benefactors.
I would not say to go thank them, but certainly, try to forgive them.  They know not what they do.
Namaste.

18:27 7/28/2003