I think it is mostly just a really bad habit defense mechanism from being anxious; kind of like guerrilla warfare? If we perceive someone as being stronger than we are or feel like another has caused us a problem and we don't know how or are unwilling to confront them directly, we get them back when we think we are stronger or feel they are not able to retaliate, etc.

If you are in therapy, I would work on identifying and expressing your feelings when you feel them, rather than having so many "I should have. . . " experiences that then go awry :-) It can be a form of negative score keeping too.

My mother wouldn't let me have another cookie before dinner so I'm not going to eat my vegetables like she wants me to.

The problem with passive aggressive behavior is that it takes place in one's head and the other person doesn't necessarily understand what is going on, just like the person behaving in that way is not fully understanding the situation either; the kid that won't do what his mother wants isn't understanding that the mother doesn't want it for herself, she wants the child to grow up big, strong, and healthy

In a sense, the temper tantrums of a 2 year old are passive aggressive and if the parent lets the child have their way to end the tantrum, they teach the child that such behavior works, but the problem is it might only work with that parent and when the child gets to school or out in other situations with other people, the child will be at a double disadvantage because such behavior won't reward the same way and the child won't have learned better ways to deal with their anger, disappointment or frustration.

Passive aggressive behavior is engaged in primarily through a misunderstanding of the dynamics of what is going on. If you want to stop the behavior in yourself, I would first train myself to stop when I feel like paying someone back for something, when I'm angry and have an "I'll show you!" reaction. The "passive" of passive aggressive is because someone does not speak up at the time of the original perceived difficulty and work on it then with themselves. The aggressive behavior may be easier to see/stop than figuring out when one is being passive and teach one's self to focus on the scary or disappointing situation at that time.

Passive aggressive behavior can be relatively minor, common, such as sulking/not talking instead of working on a disagreement with a significant other and then bringing up that incident in an unrelated argument later as ammunition; or it can be quite complex, becoming a bad habit that occurs with most other people in most other situations, that one might want professional help to untangle.