Optimistic politicians win elections, pessimistic politicians don't.
Of course, you can't radiate optimism all the time, and you have to face up to the enormous challenges which face you, but if you can be honest about the challenge AND optimistic about the result of meeting that challenge, I reckon you will usually triumph over those who radiate pessimism and negativity. It's why Reagan beat Carter. It's why Kennedy beat Nixon. It's why Thatcher beat Callaghan. It's why Clinton beat Bush. It's why Blair beat Howard.
And it is why Cameron will beat Brown.
UPDATE: You can read David Cameron's BIG SOCIETY speech HERE.