A single event can have a huge impact on your mood and outcomes. Breaking news, remarks, interactions - these can all influence your mood throughout the day. These in turn will affect the outcome of anything you try to do. Try to focus on something immediately after getting berated by someone - it will not be easy.
The example above is extreme to make a point. Most events are not black and white - they are grey with both positives and negatives. Extreme events are an exception and I should not deny myself the proper emotion. The rest of the time, I can clearly associate a positive mentality with better enjoyment and results from an event or activity. Thinking about negatives will only magnify them and leave me more disappointed.
It’s easy to be cynical and look over the positives. I made this a principle because I want to be more positive. I am aware of the effect it can have on both my own life and those around me. Each day I interact with dozens of people. They will interact with dozens of other people, and so on.
Being positive is one of the simplest choices I can make to yield outsized results.