Why do people do "passive aggression" usually? and how?

They have a mental sickness and can be very intelligent people..Once you recognize they are passive-aggressive you must be willing to be flexible with them until they get help..Don't put demands on them and know that their behavior is out of their control..And yes, they can spend time obsessing over ways to 'get even'..They best attitude to have with someone you love that has this disorder is for you to be calm..Almost like dealing with a child..