"TO ERR IS HUMAN; TO FORGIVE IS DIVINE"

This is another quote that many humans have heard at least once in their lives. I know I have heard it so many times, it inspired me to start creating my own "lessons for life", for instance "Lesson Lived is a Lesson Learned". Nethertheless, my mind is curious to understand the true anatomy in why we as humans are not forgiving creators. I will use myself as an example. I am in implosive person, mean if someone does my wrong, or hurts me i will not show it or react upon it. I will simply mark it up as a note in my memory and move on. The if that person crosses me again, it could be the next time or fifty times down the road, all the memories of negativity i have kept deep inside explode like a bunch of dynomite sticks attached to the base of a building making it tumble to the grown. That is what i will do to humans. I hold grudges like no other. Why? What makes the mind state be controlled over this type of negativity and pain? What does it stem from? Just some thoughts i had.