The problem with pessimistic expectations, such as those of the clinically depressed, is that they have the power to alter the future; negative expectations shape outcomes in a negative way. Not everyone agrees with this assertion. Some people believe the secret to happiness is low expectations. If we don’t expect greatness or find love or maintain health or achieve success, we will never be disappointed. If we are never disappointed when things don’t work out and are pleasantly surprised when things go well, we will be happy. It’s a good theory — but it’s wrong. Research shows that whatever the outcome, whether we succeed or we fail, people with high expectations tend to feel better. At the end of the day, how we feel when we get dumped or win an award depends mostly on how we interpret the event.

http://www.theatlantic.com/health/archive/2012/12/why-were-born-optimists-and-why-thats-good/266190/