Apropos nothing at all, a few definitions which have been bandied about these past few minutes here at work:
"A pessimist sees his glass half-empty. An optimist drains it, then hands it over for a refill."
"A pessimist is a man who puts All Bran on his prunes."
And my favourite:
"Definition of a pessimist: a well-informed optimist."
And just for the craic, here's a quote from Gore Vidal in which there is more than an element of truth:
"It's not enough to win - others must lose."
LATER: Well, that is what I thought it was, but it isn't. An hour or so after writing the above, I had a vague memory of giving the above quote, only to be told I was wrong. A minute ago, I got around to looking it up. The correct quote is:
"It's not enought to succeed. Others must fail", which is even better.
Vidal is also quoted as saying "No good deed goes unpunished", though whether he actually came up with it or was just passing it on, I don't know.