As an addendum:
I searched the forum for happy and sad, just to do a little statistics:
sad: 2426 thread results
happy: 6403 thread results
so of the polling, given a total of 8829 responses, 27.5% are sad, and 72.5% are happy. As far as being happy overall with their lives, Im not sure that this is totally accurate, but people using the words in their own common speech (and the frequency) is usually a decent indication about how they feel about their lives.
(sorry for the double-post)