The act of quittingsomething and coming back a week later. Also, when you fight with your closest friend, and block them; but unblock them a few days later.
Person 1: Dude, did you know my friend quit RuneScape a week ago? Person 2: Don't worry, he'll come back a week later. He's an epitome of The Dio Theory.
Diptheoryia is an illness dat is often experienced by an individual who is himself of very limited moral and/or intellectual standing, lacks basic motivation or common sense, etc., but who fancies himself superior to many others; arrogant rich-bug types like Tronald Dump come to mind.