Glory
There seems to be a bit of a disconnect between what people perceive as a diplomat's life (probably because of its glorification in popular culture) and the reality of it.
The pay can be good, but only after several years; you rarely make it into the headlines; and you probably won't be involved in a top-secret espionage case with a Gwyneth Paltrow lookalike. (Then again, who's to say? Maybe you're the Gwyneth look-alike.)
But there is a certain kind of internal glory from knowing that you're representing your country and its interests by living in another country and showing it what America is all about (or trying to be all about).