Jeb Bush's "stuff happens" response to the Oregon mass-shooting confirms that, like the rest of his family, he is a colossal douche bag.
Beyond that, responses of the "shit happens" variety are intended to close off debate and paper over all the problematic assumptions and structural issues that caused the problem being discussed.
Yes, there are cases where shit just happens for no reason, or for reasons no one could do anything about. In this case, though, there are very clear reasons why shit happened. Bush's response is intended to paper over the fact that our society is awash in guns. It is a rhetorical trick intended to make us just accept that as a fact of life and move on, regardless of how many people get killed as a result.