From Emmet Fox's The Sermon on the Mount.
"Thinking about sickness or disease is only one of the two factors that produce bodily ailments, and it is usually the less important. The other, and more important factor, is the entertainment of negative or destructive emotions, although this fact seems to be very little understood . . .So important is it, however, that it is simply imposssible to insist too strongly upon the fact that most bodily ailments are caused by the patient's allowing destructive emotions to hold a place in his mind. It cannot be too often repeated that to entertain feelings of anger, resentment, jealousy, spite, and so forth, is certain to damage your health in some way or other, and quite likely to damage it very severely indeed. Remember that the question of the justification for such feelings does not arise at all. It has absolutelying nothing to do with the results, for the thing is a matter of natural law.
A woman said: "I have a right to be angry," meaning that she had been a victim of very shabby treatment, and that she consequently possessed a kind of license or special permit to hold angry feelings without their natural consequences upon the body following. This, of course, is absurd. There is no one to give such a permit and if it could be done--if general laws could ever be set aside in special instances--we should have, not a universe, but a chaos. If you press the button, from no matter what motive, good or bad--to save a man's life or to murdr him--the electric bell will ring; because that is the law of electricity. If you drank a deadly poison inadvertently, you would die or at least seriously damage your body, because such is the law. You may have mistakenly supposed it to be a harmless fluid, but that would make no defference because the law takes no account of intentions. For the same reason, to entertain negative emotions is to order trouble--primarily physical trouble, and also trouble in general--quite independently of any seeming justifications which you may suppose yourself to have...
To indulge in a sense of execration of anyone (quite irrespective of any question of deserts, or otherwise, in the object of your condemnation) is certain to bring trouble upon your own head proportionate to the intensity of the feeling you entertain, and the number of times or minutes that you devote to it."
I believe this is absolutely true but not known or understood by the majority of people today. Even when we know it, it is difficult to put into practice because we feel we have 'rights'.
One way to help ourselves stop trying to exercise our 'rights' to be angry, or offended, or resentful is to change the way we see the person who has caused us harm. For me, another quote from this book which I posted earlier, helps much. "Let us be merciful in our mental judgments of our brother, for, in truth, we are all one, and the more deeply he seems to err, the more urgent is the need for us to help him with the right thought, and so make it easier for him to get free. You--because you understand the power of the Spiritual Idea, the Christ Truth--have a responsibility that others have not; see that you do not evade it. When his delinquency comes to your notice, remember that the Christ in him is calling out for help to you who are enlightened--so be merciful..."