From: The Monk who Sold his Ferrari

We might not be able to control the weather or the traffic or the moods of all those around us. But, we most certainly can control our attitude towards these events. We all have the power to determine what we will think about in any given moment. This ability is part of what makes us human.
There is no such thing as objective reality or 'the real world'. There are no absolutes. The face of your greatest enemy might be the face of my finest friend. An event that appears to be a tragedy to one might reveal the seeds of unlimited opportunity to another. What really separates people who are habitually upbeat and optimistic from those who are consistently miserable is how the circumstances of life are interpreted and processed.