people usually have regret after being sincere with their actions because they were either taken advantage of or "abused". Regret also applies to those who have not been sincere or fair with their actions. Regret usually comes after one has made a sacrifice in the name of, or one has neglected or abused whatever sacrifices in the name of. Also you can have regret after exhibiting repulsive behaviors, making bad decision as well as assumption inregards to someone or something and in return you're no longer respected or acknowledged because of your actions.
At the end of the day regret should make you stronger, whether you have been negative or positive. A lesson in which both parties should learn they should not do X,Y,Z ever again!
P.S. Then again this sounds good on paper, but the average person either refuses or has no interest in "learning from their mistakes" Interest only rises when it's too late!