Why so serious?

The Joker does things just for the sake of doing them. He has no plan or motive, and he doesn't care about the consequences. He just thinks of something and does it without giving it much thought. So, why so serious?
JUST ENJOY IT AND SHAKING YOUR FINGER !!!

Characters (left to right):
Joker - Batman #1 (Spring 1940)
Wonder Woman - All Star Comics #8 (Jan. 1941)
Green Lantern - All-American Comics #16 (July 1940)
Batman - Detective Comics #27 (May 1939)
Cyborg - DD Comics presents #26 (Oct. 1980)
Superman - Action Comics #1 (April 1938)