Perhaps at times, it isn't sensible to dredge up old wounds, and a person should just let things go. But one also gets that impulse to make the other party understand what he/she did was inappropriate and wrong, and ask him/her not to do it again. And so the internal debate begins...
Of course, the person who writes the polite letter expressing concern or dissatisfaction doesn't get the final say. The recipient of the letter may take it in a good way and respond with an equally thoughtful reply, allowing for mutual understanding, enhanced respect, and a positive outcome. But more likely, the recipient refuses to acknowledge wrongdoing and lashes out in a defensive manner, creating more ill-will and pain.
The question is this: if there is currently no conflict, is it better to just leave well-enough alone? Should we let old unresolved issues rest undisturbed, or is it important to address them before an acquaintance/friendship/professional relationship can proceed?
Are we taking the high road by letting go of past provocations? Or should we worry about coming across as weak-kneed because we haven't (yet) actively responded to unfair and abusive actions? (By the way, letting go probably means actually letting go, not just ignoring it and holding an incident in reserve for future use. Then again, maybe one step toward letting go is to first express it).
Perhaps weakness is dispelled and control regained when you can frame an issue in your own terms -- expressing your thoughts and concerns. What of honesty? To oneself? To others?
Can a personal or professional relationship be healthy if there are issues from the past that remain unaddressed? What if one party has transcended the issue, but the other party merely forgets that it happened? Is it the responsibility of either party to make sure the other knows of his/her transgression and the impact it has had on others? What happens if Person X doesn't know what he did was wrong -- is it our job to inform him?
We can also frame the issue yet another way: When we let Person X know that his actions were detrimental or harmful to others, we are simultaneously protecting ourselves.
Do we hope for the best, but take no action and simply prepare for the worst (interpersonal outcome)? Can explicitly raising the issue be useful for signaling to the other person that a problem exists -- that contrary to his/her beliefs, all is not well?
Dispatching a message can be done in a civil manner. And though there may be further tussling, perhaps at the end of the day, it is something one simply has to bear, and from which one rises again.
But one wonders: are there other diplomatic strategies for approaching these matters? Is it a matter of the heart or the head?