Putting on the right lense
The lenses we use to view events in our lives can greatly affect our perception of those events.
I'm sure you've seen pictures of galaxies, with their magnificent colors and shapes. The naked eye will only see a black space because it is only capable of seeing visible light. But with the right set of tools, you can "see" much more.
If we only had to rely on our naked eye, our default lens, we wouldn't make much progress. We can't see atoms that are very small nor can we see the sun that is very bright. However, here too, with the right set of "lenses" (microscopes and a piece of Obsidian), we can observe both.
The same can be applied to our daily lives when faced with certain events. The world around us is neutral. It us who "see" an event as good or bad, and we do this by looking at it with a given lens. These aren't actual physical lenses, but it follow the analogy above - the choice of lens determines what we can or cannot see.
As we go through life, we are faced good and bad times. We hope to see more of the good and less of the bad. This hope stems from the belief that we have no control on what we will face. While that is true about most of life, we have some control on the choice of lens we use.
Take death as a example. It can been seen through a lens of sorrow for the person is no more. Or through the lens of joy for the impact that person had on the lives of many. By taking over the choice of lens, we can be less worried and anxious about facing the uncertainties of life.