I wrote yesterday that Depression has roots in one's genes. However, that only indicates a susceptibility for someone with "depression-genes" to become depressed; you can do things to your environment to alter that effect.
So while there are people who naturally see their glass half empty, with practice, it's possible to stray away from that mindset.
Now of course, everyone gets into the rut of thinking negatively every now and then. Whenever that's the case, have a beer with a good mate. I'm sure we can all agree that beer comes in pairs.