Why Does the Kookaburra Laugh?

As a child, you probably learned in school that the kookaburra is a bird native to Australia and that it has a really funny name. 

You might even have learned the kooky little kookaburra song:

Kookaburra sits in the old gum tree.
Merry, merry king of the bush is he.
Laugh, kookaburra!
Laugh, kookaburra!
Gay your life must be.

But what else do you know about this chuckling feathered friend from the land Down Under?
Why Does the Kookaburra Laugh?