This line is probably going down in history as the most famous
and memorable quote from the new Batman movie 'The Dark Knight'.
Spoken by the late Heath Ledger as The Joker.
The tagline was also used in the production company's marketing campaign for the movie.

In every day use it can be used to tell someone to lighten up.

A quote from the Joker in The Dark Knight, turned in to an internet fad/meme. Usually used on angry, emotional people who tend to go overboard or not understand jokes.

A famous quote by the Joker from The Dark Knight, the Batman movie starring Christian Bale and the deceased Heath Ledger.

What The Joker wants to know.

A very well known statement created by the hottest villain of all time.

In reference to the Joker in Batman, "Why so serious?" is a phrase for somebody to use who is openly stating that they are wearing excess amounts of makeup.

Originally a Batman marketing campaign, "why so serious" can be used as a response to someone taking themselves or a particular topic too personally or seriously.