What does "Why so serious" mean?

Question

Is everyone quoting that picture of Miley Cyrus, or does this phrase mean something?

Answer 1

Yeah it's a phrase that's been around a long time, just popular now with the movies and stuff.

Answer 2

Please tell me you are kidding?! That phrase has been around since before Miley Cyrus was born.

Answer 3

It's from The Dark Knight actually.  It's a line said by The Joker (Heath Ledger) to someone else.  Since it's such a great movie (or so they say) people are quoting it.  I personally have no interest in seeing it.