Part of DBT (Dialectical Behavior Therapy) is learning about the core emotions and their functions, so that later one is able to spot a malfunction. It makes you feel very stupid at the beginning, when the therapist asks you to explain what is fear, what is guilt, what is happiness, but indeed for people with BPD trouble with emotions is big part of the disorder.
Let us take anger now:
The biological function of the anger is to motivate you to make some action that will improve your situation.
But what happens if you are neglected and offended by your parents as a child, and start feeling anger basically constantly? It becomes such a standard emotion of everyday life, that you stop even realising that you are angry. And because acting out and appearing angry constantly is not quite socially accepted, you start to try to ignore it. As long as you can bear it you stop reacting to it. It becomes your new baseline.
What is the result? You do not tell people when they start crossing the line, because you do not even realise when that happens. You constantly feel bad and irritated but you cannot tell why. You feel constantly abused as your boundaries are constantly violated by the people around you - which is expected as you don't show people where your boundaries are. And while feeling so hurt you know well that, objectively, no one is doing you any harm. You start asking yourself "am I crazy?".
You start creating double standards. You know that people around you hurt you constantly, yet you cannot pinpoint why that is. So you start acting as if everything was fine, inside being torn apart. You claim you have friends while the people you call that are the main sources of suffering for you. You are so used to not saying aloud what you really think (not that you know it yourself anymore) that in critical situations you will also not speak up. When you see your child being beaten up by your husband, you will say nothing. When someone you know is doing a crime and hurting someone else, you will do nothing. When someone at work steals your project, you will also do nothing. You live your anger only in your head, but you stop taking actions based on it.
In such situation the initial function of anger is lost. It no longer has the power to motivate you to make actions in the direction of improving your life. It's a disfunction. Instead of helping it starts to get in the way. It makes you grumpy and disappointed.
Does that sound familiar to anyone? It is just my theory of recent days, but it makes sense to me..