Why have a positive mindset?

We all know we should have a positive mindset, but how many of us actually succeed in keeping it?
The first negative conversation / news article / comment steal our joy in mere seconds!

Why do we tend to follow the masses into the trap of believing that negativity is okay? The influence a negative mindset has is severe and totally underestimated. Feeling ups and downs regularly, not having enough energy, thinking negative punishing thoughts, are only a few examples of the consequences of a negative mindset.