Breakup Letters

In years past, when a girl no longer had interest in a boy, she would set to writing what was referred to as a "Dear John" letter. Anyone living in the past few decades were well aware of this term and the meaning behind the phrase "He got a Dear John letter".
While the exact origins of this phrase have not been determined, popular belief is that is started during World War II by American women who weren't patient enough to wait for their military partners to return. Whether they wanted to relive themselves from the worry and waiting involved with a military deployment, or they just found new interest in what was "closer to home" could be argued.
Using the name "John" is believed to be because of the name being so common at the time the phrase was coined. It is also frequently used to refer to an anonymous person, as in "John Doe" or "John Smith". "Dear John" also has an impersonal feeling to it as opposed to "Johnny", "Darling", or even "Dearest John" when used to begin a letter.