Why did I laugh tonight? No voice will tell
  No God, no demon of severe response
  Deigns to reply from heaven or from hell
  Then to my human heart I turn at once:
  Heart, thou and I are here, sad and alone,
  Say, why did I laugh? O mortal pain!
  O darkness! darkness! Forever must I moan
  To question heaven and hell and heart in vain?
  Why did I laugh? I know this being's lease
  My fancy to it's utmost blisses spreads
  Yet would I on this very midnight cease
  And all the world's gaudy ensigns see in shreds
  Verse, fame and beauty are intense indeed
  But death intenser, death is life's high meed.

One of my favourites…

What I love about this poem is the fact that its so uncharacteristic of Keats - so dark and feverish, quite a change from his usual tranquility: Keats is still ceasing upon the midnight, but no longer ‘with no pain’. Plus of course it’s a poem that cries out to be read aloud, the repetition of the original question adding a dramatic soul-searching intensity: almost like the sound of a man drawing in air between fits of pain. But that’s not all that makes it dramatically intense - there’s also the alternation between anger and anguish, between god and demons on the one hand, and his own heart on the other, all of it ending with an almost heroic disillusionment that one (or at any rate I) associates so much more with Shelley than with Keats. Altogether a wonderful poem that shows off the more savage side of Keats to perfection.

— Aseem

[I’ve added in a few links - m.]

Links

Here’s an analysis of the poem:
http://icdweb.cc.purdue.edu/%7Efelluga/Apaper241F98.html

And an excellent Keats site, complete with biography:
http://www.john-keats.com/

(The above site also has a ‘vote for your favourite Keats poem’ poll - worth taking a second or two on)