The nursing of grudges is the response to inner impulses based on beliefs within the being which give rise to strong feelings on the matter. When one is triggered by seeing an old enemy or put in a situation where they are being challenged unfairly or treated unfairly, there may well be a strong inner emotional reaction. Many times there is little one can do to change another person to be more understanding, tolerant, or forgiving, and the same is true of institutions that have arbitrary rules so one is left with little recourse many times to make life fairer. Because there is a lack of control of the world, one can only control oneself if they learn how to do this, but it is much more difficult to change things around you. So it is the inner beliefs that one can expect trouble, one can expect ill-treatment, and one can expect to be left unhappy in the end, which will give rise to an emotional reaction whenever there is a reminder of prior difficulty, and if there is an automatic hatred, that is what constitutes a grudge by definition, an automatic knee-jerk response to make hatred uppermost in the mind by the presence of an opposing force.
It is called “nursing” because people who are wise understand there are ways to work through such dilemmas, to find a path for healing and forgiveness, to get out from under the burden that grudges represent from being in the way of happiness. In a sense, it reinforces the negativity from the opponent because it becomes self-applied again and again through having a strong emotional reaction within the victim, and that adds to the injustice of the situation. But in this case, the victim is digging their own grave, so to speak, by cultivating an inner pain response that happens over and over and over again and is of their own doing. This is why it becomes a karmic dilemma because, in a sense, it is something chosen, if only by default. Wisdom to seek healing is the antidote.