She or he’d cheated on them, betrayed them somehow, dumped them with callous disregard to their feelings, devoured their soul or otherwise played games with their heart and now, as the wronged party, they deserved satisfaction.
Usually what they want is, ultimately, vindication.
But whether you start burning that energy by working out or scribbling angry manic poetry or inventing the core of what ultimately becomes a world-dominating social networking platform, you want to find a channel and productive output for all of that energy.
It helps you process your pain faster and, ultimately, is healthier than sitting around stewing in your own resentment or eating your feelings instead.
I repressed it as hard as I could and refused to even admit it was there.