Black and white, Yin and Yan, Pessimists and optimists - all this is a universal balance. But what is the balance?
As you gave an example above - for one person a glass is half empty and for another glass is half full. People are different and it has always been so. But there is a saying - "when we have not value, but lose and weep." Its meaning is that only after losing something, we begin to appreciate this thing or this person. Everything is relative.
Well, too, the rule applies to the world around us. For example, you can not always enjoy life, somehow negative events or life difficulties occur. The whole point is how you perceive them. If the dark side prevails in your soul and your mind - you will panic and blame the situation. If you look at the world with optimism - you will take a lesson from this situation and take it calmly.
We can observe the balance in everyday life, for example, a person who undeservedly received large riches more often just as easily and quickly with them is forgiven. The person who commits the evil, one way or another will be punished for it. Either society or karma.
The small area that goes to the dark side as if tells us that the preponderance will always be on the white side. Good thoughts and healthy optimism - that's what matters.
Goodnight:)