Revenge

"You get what you give," Gregg Alexander once said (and sung). Truth be told, he gave the world a great music album, but maybe his love was too radical. The world only got what he gave: a one hit wonder.
When John A. Nevin coined the term "behavioral momentum" he referred to the well known behavioral patter of continuing what one (or a rat) is doing in order to get something, even when that something one (or a pigeon) is willing to get is no longer available.
Fun things happen when one uses the laws of physics in the psychological world. Newton said (not with these words) that for every action there is a corresponding, equal, opposite, reaction. This does not mean that if a ball hits the ground it will bounce back; it means that the ground will suffer a deformation as the ball does when it hits it (whether it bounces back or not).
Newton was a genius, standing at the shoulders of giants. I don't know whether Newton was trying to reveal God's truth, understand the nonliving world, or finding out why humans behave as they do; but I bet he had a great understanding of the three.
Newton hated Leibniz (they hated each other, actually). Maybe if Leibniz had been static he would not have moved Newtons feelings. Maybe if Newton had been rewarded by God he would have continued developing his calculus system (without caring about the other systems). Maybe emotional reactions are unavoidable.
When you, reader, say "thanks," do you do it expecting something back? When you, reader, say "I'm sorry," do you do it to relief yourself, or someone else? Revenge is an emotional metaphor of Newton's third law (or vice versa); so is gratitude, so is forgiveness, so is almost any human "value" involving two or more people (or a ball and a floor).
A little experiment, less than 5% of the world's population are able to do this. Think of someone you used to like that you currently hate or dislike a lot. Give him/her a phone call, an email, or a SMS saying "I'm sorry." See what happens. Although I'm making up more than 5% of my statistics, I bet I'm not far from the actual value.
Do it; if Newton and Gregg Alexander are right, you'll get what you give.