As most events and situations have both a positive and negative aspect, there is a danger of focusing all our attention on the negative side

"As most events and situations have both a positive and negative aspect, there is a danger of focusing all our attention on the negative side. Obviously, we should not delude ourselves and see only what is good, but neither should we dwell on what is bad, for then we no longer even see the good.

You’re thinking, ‘Oh, we already know all that.’ So, put it into practice then, if you know it! Watch yourself, and you will discover how often you forget what you know, giving way to unhappiness and pessimism. And then, not only do you fail to reason correctly, you also prevent your soul from opening out and taking flight. This is how people destroy themselves spiritually, even physically. Yes, why do we say colloquially, when someone is anxious and dissatisfied, that worry is ‘eating them up’ or ‘gnawing away at them’?…"