Passive aggression is just that – aggression (or anger) that is passive (or hidden). The passive-aggressive person learned to conceal their rage from the very start, in a home where it was not safe to express such frustration. This home could have been unsafe for many reasons. There could have been drug or alcohol-addicted parents or parents whose gender roles were heavily enforced (for example, the wife was not allowed to express her frustration against the husband).
As the child (later passive-aggressive) was taught to suppress and deny their feelings, they sought ways of getting around that. They found other channels to express themselves, passively resistant ways. This is how sabotage (covert behavior, forgetting, ambiguity, chaos creation) and retaliation (overt punishment, eye for an eye, “justified” abandonment or abuse) are learned.
Children who grow up in this environment are never taught to find healthy ways of expressing anger. To the people who are in charge of these children, anger is not supposed to exist in the first place. Thus, the child continues to adulthood, still suppressing anger and venting it in vindictive ways.
So, what are the ways of healthily expressing anger? How do we teach our children to identify feelings and accept them as part of themselves? How do we show them that there is no need to run away?
The child needs to know they can say "I am angry." They need to be taught the vocabulary for this.
When they do, appreciate their voicing it. "I'm glad you shared this with me."
Ask them to stay in that feeling. "Why don't we sit down and talk about why you're angry?"
Why are they angry? What do they need that they don’t get? “I’m angry because they left me alone in the house.”
Validate those feelings, and let the child know those feelings are theirs, they are human, and they are OK. "Well, it's normal to feel sad when you are alone."
But now - follow that up with teaching the child that there is no need to become distraught. Instead of jumping to demand an immediate solution ("Don't ever leave me alone again!"), the child needs to learn the value of owning their feelings and finding ways of helping themselves feel better ("Playing with my toys makes me feel happy when I'm lonely").
This manner of handling emotion is an extremely important part of human development. If we make others do things so we feel better ("If you never leave me alone, then I won’t feel sad"), we create this idea that feeling sad is bad and that we should do everything we can to block and prevent feeling exactly that...
Instead, as a healthy human being, the child should learn that emotions are healthy and valid and that they just need to listen to themselves and take care of themselves when they are upset. Instead of demanding you stay (using other people to make the feeling stop), they can learn what to do and what to say to themselves to reframe being lonely as a normal and not a catastrophic situation, helping themselves accept sad or angry feelings as normal and transitory stages. This is the most basic lesson in self-sufficiency.