People who have something good happen to them should be happy and fulfilled, have some purpose in their lifel and prove that life is good and they can spread the goodness around.
But some of them blow it because they can't control their reactions to their good fortune. It all turns to shit and their life is therefore more crappy than it would have ever been if they didn't have that really good luck happen to them in the first place.
Someone like that is a shitbritches.
For a while there, Britney Spears was a pure shitbritches.