I think one develops passive-aggressive reactions because one is not allowed originally to be active-aggressive! It is sort of a second-best way of handling difficulties, if the first-best is not allowed. As an adult one can learn to be "active-aggressive" in the privacy of one's own mind, if nowhere else, but this is a skill that is not easy to learn if one was not allowed as a child. You might encourage someone with this behavior by letting them know that you understand their thinking, or their wishes, even if they cannot safely carry them out often in actions.